THI*  DOOK  II  A  PART 
lOFTHE  LIBRARY  OF 


H  BARBARA  STATE  COUEG 

3010 


MINERALS 

AND 

HOW  TO  STUDY  THEM 


DANA'S 
SERIES  OF  MINERALOGIES 

System  of  Mineralogy 

Sixth  edition,  entirely  rewritten.  With  Appendices  I  and 
II,  completing  the  work  to  1909,  1333  pages  and  more  than 
1400  illustrations;  and  with  the  Third  Appendix,  by  William 
E.  Ford,  Professor  of  Mineralogy  in  Sheffield  Scientific  School 
of  Yale  University,  completing  the  work  to  1915,  87  pages. 
Complete  volume.  6f  by  10,  half  leather. 

A  Text-book  of  Mineralogy 

With  an  extended  Treatise  on  Crystallography  and  Physical 
Mineralogy.  By  Edward  Salisbury  Dana,  Late  Professor 
Emeritus  of  Physics  and  Curator  of  Mineralogy,  Yale  Uni- 
versity. Fourth  edition,  rewritten  and  enlarged,  by  William 
E.  Ford  (1932).  851  pages,  6  by  9,  1089  figures.  Cloth. 

Minerals,  and  How  to  Study  Them 

A  book  for  beginners  in  Mineralogy.  By  the  late  E.  S.  Dana. 
Second  Edition.  380  pages,  5  by  7^,  319  figures.  Cloth. 


Manual  of  Mineralogy 

For  the  Student  of  Elementary  Mineralogy,  the  Mining 
Engineer,  the  Geologist,  the  Prospector,  the  Collector,  etc. 
Fourteenth  edition,  entirely  revised  and  rewritten,  by  Wil- 
liam E.  Ford.  476  pages,  5  by  7j,  360  figures,  and  10  plates. 
Cloth,  or  flexible  binding. 


QUARTZ    CRYSTAL-NORTH    CAROLINA 
Two  fifths  natural  size. 


MINERALS, 


AND 


HOW    TO    STUDY    THEM 


A  BOOK  FOR  BEGINNERS  IN  MINERALOGY. 


THE  LATE  EDWARD  SALISBURY  JDAN  A 

YALE  UNIVERSITY,  Atw  HAVB.N, 

Author  of  A  Text-book  of  Mineralogy,  Sixth  Edition  of 
Dana's  System  of  Mineralogy,  etc. 


TTCMb  more  tban  300  Illustrations. 


SECOND  REVISED  EDITION 
TOTAL  ISSUE,  TWENTY-SEVEN  THOUSAND 


NEW  YORK 

JOHN    WILEY   &    SONS,    INC. 
LONDON:  CHAPMAN  &  HALL,  LIMITED 


COPYKIGHT,  1895, 
BY 

EDWARD  S.  DANA 


AU  Rights  Reserved 
This  book  or  any  part  thereof  must  not 
be    reproduced    in    any   form    without 
the  written  permission  of  the  publisher* 


PRINTED  IN  u.  e.  A. 


12/35  PRESS  OF 

BRAUNWORTH  8c  CO..  INC. 
BOOK  MANUFACTURERS 
BROOKt-YN.  NEW  YORK 


QE. 


to 


PEEFACE. 

THE  author  has  occupied  some  hours,  which  could  not 
be  devoted  to  more  serious  labor,  in  preparing  this  little 
book,  in  the  hope  that  it  might  serve  to  encourage  those 
who  have  a  desire  to  learn,  about  minerals,  and  also  to  in- 
crease the  number  of  those  whose  tastes  may  lead  them  in 
this  direction.  He  shares  with  most  teachers  at  the  pres- 
ent time  the  conviction  that  the  cultivation  of  the  powers 
of  observation  is  a  most  essential  element  in  the  education 
of  young  people  of  both  sexes;  he  believes,  further,  that 
no  subject  is  better  fitted  to  accomplish  this  object  and  at 
the  same  time  to  excite  active  interest  than  that  of  Miner- 
alogy. The  attempt  has  been  made  to  present  the  whole 
subject  in  a  clear,  simple,  and,  so  far  as  possible,  a  read- 
able form  without  too  much  detail  and  at  the  same  time 
without  cheapening  the  science.  As  the  understanding  of 
the  different  parts  of  the  subject  requires  some  preliminary 
knowledge  of  physics  and  of  chemistry,  a  little  elementary 
matter  in  these  departments  has  been  introduced. 

Much  attention  has  been  given  to  the  illustrations,  most 
of  which  have  been  made  expressly  for  this  book;  others 
(reduced  in  size)  are  taken  from  the  sixth  edition  of  the 

System  of  Mineralogy  (1892) ;  several  have  been  borrowed 

iii 


iv  PREFACE. 

from  Tschermak's  Mineralogy,  and  one  from  a  work  by 
Baumhauer.  The  correct  representation  of  real  crystals 
and  of  the  actual  specimens  from  the  cabinet  is  a  difficult 
matter,  and  in  this  the  author  has  been  so  fortunate  as  to 
secure  the  services  of  the  skillful  wood-engraver  Mr.  TV.  F. 
Hopson  of  New  Haven.  Any  suggestions  which  would 
tend  to  give  this  volume  greater  accuracy  or  usefulness 
will  be  always  gratefully  received. 

NEW  HAVEN,  July  1,  1895. 


TABLE  OF   CONTENTS. 


CHAPTER  PAGH 

I.  MINERALS  AND  MINERALOGY:  INTRODUCTORY  REMARKS..  1 
II.  SOME  PRELIMINARY  HINTS  AS  TO  How  TO  STUDY  MIN- 
ERALS   8 

Suggestions  about  making  a  Collection 11 

III.  THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE 14 

The  General  Characters  of  Crystals 14 

The  Systems  of  Crystallization 21 

I.  Isometric  System 22 

II.  Tetragonal  System 31 

III.  Hexagonal  System 36 

Rhombohedral  System 39 

IV.  Orthorhombic  System 41 

V.  Monoclinic  System 44 

VI.  Triclinic  System. ...  45 

Irregularities  of  Crystals 48 

Distorted  Crystals 48 

Pseudomorphs 55 

Groupings  or  Aggregations  of  Crystals „ .  56 

Twin  Crystals 57 

Parallel  Grouping 60 

Irregular  Grouping 62 

Structure  in  General 63 

IV.  THE  OTHER  PHYSICAL  CHARACTERS  OF  CRYSTALS 70 

1.  Characters  depending  upon  Cohesion 70 

Cleavage 70 

Fracture 73 

Hardness  and  Tenacity 74 

v 


Tl  TABLE   OF  CONTENTS. 

CHAPTER  PAOK 

2.  Specific  Gravity  or  Relative  Density 79 

3.  Characters  depending  upon  Light 88 

Luster 88 

Color 90 

Transparency 92 

Other  Optical  Characters 93 

4.  Characters  depending  upon  Heat 95 

5.  Characters  depending  upon  Magnetism 96 

6.  Characters  depending  upon  Electricity 96 

7.  Taste  and  Odor 97 

V.  THE  CHEMICAL  CHARACTERS  OF  MINERALS 'J9 

The  Chemical  Elements 100 

The  Chemical  Formula,  etc 104 

Kinds  of  Chemical  Compounds  among  Minerals 109 

Percentage  Composition 116 

Classification 118 

VI.  THE  USE  OF  THE  BLOWPIPE 121 

1.  General  Description  of  Apparatus 121 

2.  How  to  Use  the  Blowpipe 127 

3.  Examination  in  the  Forceps 130 

4.  Use  of  the  Platinum  Wire 186 

5.  Use  of  Charcoal 140 

6.  Use  of  the  Closed  and  Open  Tubes 147 

7.  Chemical  Examination  by  Acids  and  other  Reagents.   153 

VII.  DESCRIPTION  OF  MINERAL  SPECIES 158 

VIII.  THE  DETERMINATION  OF  MINERALS 339 

APPENDIX 365 

GENERAL  INDEX 369 

IXDEX  TO  MINERAL  SPECIES 878 


MINERALS,  AND  HOW  TO  STUDY  THEM, 


CHAPTER  I. 
MINERALS  AND  MINERALOGY. 

WE  are  to  learn  about  minerals  and  how  to  study  them; 
but,  before  we  can  fairly  begin,  we  must  understand  clearly 
what  substances  we  may  call  minerals,  and  what  specimens 
have  a  right  to  a  place  in  the  collection  that  every  one 
who  wishes  to  become  a  mineralogist  must  make. 

We  all  know,  in  the  first  place,  that  minerals  are  the 
materials  out  of  which  the  earth  is  built,  and  we  often  hear 
that  division  of  nature  to  which  they  belong  called  the 
Mineral  Kingdom,  in  distinction  from  the  Animal  and 
Vegetable  Kingdoms,  which  embrace  the  animals  and 
plants  which  live  and  grow  upon  the  earth's  surface. 

And  here  it  is  important  to  realize  how  little  we  can 
know  by  actual  contact  and  direct  observation  about  this 
earth,  though  we  live  upon  it.  It  is  possible,  indeed, 
to  measure  its  size  and  shape,  to  find  out  its  density  as  a 
whole,  to  study  its  surface  features  and  the  changes  which 
they  have  undergone;  but  of  the  materials  of  which  it  is 
made  we  can  know  little  beyond  those  which  form  the 
surface  upon  which  we  walk.  The  miner  digs  down  a 


2  MINERALS,    AND   HOW   TO   STUDY   THEM. 

little  distance,  and  the  artesian-well  borer  goes  down  still 
deeper,  and  we  may  have  a  chance  to  examine  the  spec- 
imens that  their  work  brings  up;  or  perhaps  we  can  go 
down  with  the  miner  and  see  them  in  place.  But  th«> 
deepest  mines  descend  to  less  than  three  quarters  of  a  mile, 
and  though  this  seems  deep  to  one  who  is  let  down  a  shaft 
in  a  bucket,  it  is  but  a  little  way  compared  with  the  whole 
distance  to  the  earth's  center,  which  would  require  a  journey 
of  nearly  4000  miles.  Even  the  deepest  artesian-well  bor- 
ings hardly  go  down  to  the  depth  of  one  mile. 

Our  knowledge,  to  be  sure,  is  increased  a  little  by  the 
fact  that  we  find  now  on  the  surface  of  the  earth  rocks 
made,  as  we  have  reason  to  believe,  of  materials  brought 
up  in  a  molten  condition  from  great  depths  below.  This 
is  true  of  the  lava  thrown  out  by  a  volcano,  and  of  such 
igneous  rocks,  for  example,  as  form  the  Palisades  along  the 
Hudson  River;  and  these  occurrences  give  us  some  idea  as 
to  what  kinds  of  matter  there  are,  and  in  what  condition, 
far  below  the  surface.  Further,  we  are  able  to  weigh  the 
entire  earth,  too,  and  find  what  its  density  is;  and  as  this 
is  nearly  twice  as  great  as  that  of  the  rocks  on  the  surface, 
it  gives  a  suggestion  as  to  the  heavy  nature  of  the  mineral 
material  that  must  make  up  the  interior. 

Thus  the  mineralogist  is  limited  to  the  study  of  the  lit- 
tle part  of  the  crust  of  earth  which  he  can  reach  with  his 
hammer;  and  he  cannot  extend  his  collection  much  be- 
yond this,  unless  indeed  he  takes  in  some  of  those  rare 
visitors  from  outer  space — called  meteorites — which  once 
in  a  while  tumble  down  to  the  earth,  usually  with  a  bright 
light  and  loud  explosion. 


MINERALS   AND   MINERALOGY.  3 

Now  what  does  this  study  show  of  the  hard  rocky  mate- 
rial of  which  the  earth,  so  far  as  we  can  examine  it,  is  made 
up;  for  example,  of  the  sand  of  the  seashore,  the  granite, 
the  trap,  the  slate  and  marble  of  the  hills  ? 

We  find,  in  the  first  place,  that  it  in  general  consists  of 
different  kinds  of  substances,  each  one  having  certain 
peculiarities  or  characters  of  its  own,  by  which  it  can 
always  be  recognized;  and  it  is  to  each  of  these  individual 
kinds  that  the  name  MINERAL  is  given. 

Thus,  more  particularly,  the  sand  of  the  seashore  can  be 
separated  without  much  difficulty  into  various  sorts  of 
grains,  each  kind  alike  in  chemical  substance,  as  the 
chemist  can  prove  in  the  laboratory,  and  with  certain 
characters  of  hardness,  density,  luster,  and  color  of  its  own, 
which  enable  us  after  a  little  practice  to  distinguish  the 
different  kinds  with  comparative  ease. 

Most  of  the  grains  are  alike  clear  and  glassy,  hard  enough 
to  scratch  glass,  and  as  we  learn  to  know  them  better  we 
call  them  quartz.  There  are  also  black  grains;  some  of 
these  are  heavy  and  jump  to  a  magnet,  and  often  they  are 
sorted  out  by  the  waves  into  little  rifts  on  the  white  sand ; 
these  are  called  magnetite,  or  magnetic  iron.  There  are 
other  black  grains,  too,  which  the  magnet  does  not  attract, 
perhaps  some  red,  glassy  ones  which  are  fragments  of 
garnets,  and,  it  may  be,  still  others,  depending  upon  where 
the  sand  comes  from,  and  what  kind  of  rock  has  been 
ground  up  by  nature's  mill  and  sorted  out  by  the  water  to 
make  the  sand. 

If  a  piece  of  granite  is  taken,  here  too  it  is  possible  to 
distinguish  several  kinds  of  mineral  substances,  though  it 


4  MINERALS,    AND   HOW  TO   STUDT    THEM. 

is  not  quite  so  easy  to  separate  them.  There  are  hard 
glassy  grains  with  irregular  surface,  which,  like  the  greater 
part  of  the  sand-grains,  are  quartz.  There  are  white  or 
yellow  or  pale  flesh-red  fragments,  also  hard,  though  not  so 
hard  as  the  others,  but  which  are  sure  to  show  one  or  two 
smooth  surfaces  of  fracture:  these  are  feldspar.  Then 
there  is  the  mica,  more  easily  recognized  still,  which  is 
either  nearly  white  and  silvery,  or  black  (and  sometimes  both 
kinds),  and  which  with  a  touch  of  the  knife  separates  into 
very  thin  scales  or  leaves.  Besides  these  there  may  be  a 
little  coal-black  tourmaline,  some  bright  red  garnets,  and 
other  kinds  which  we  shall  learn  later.  If  a  cavity  or 
open  space  in  the  granite  can  be  found,  here  it  is  often 
possible  to  find  the  same  kinds  of  substances,  only  larger 
and  more  distinct  and  very  likely  in  the  regular  form 
which  are  called  crystals. 

If,  instead  of  a  coarse-grained  rock  like  granite,  we  ex- 
amine a  fine  compact  one  such  as  the  trap-rock  of  the 
Palisades  on  the  Hudson,  it  very  probably  appears  all  alike 
to  the  eye;  but  if  we  crush  some  of  it  to  powder,  the  magnet 
will  pick  out  some  magnetic  iron,  as  from  the  seashore  sand. 
Or  the  skillful  mineralogist  may  make  a  slice  thin  enough 
to  be  transparent,  so  that  he  can  study  it  under  the  micro- 
scope, and  then  recognize  a  variety  of  different  minerals. 
In  seams  and  cavities  in  these  rocks  other  sorts  are  often 
found,  not  like  those  in  the  solid  rock. 

Sometimes  we  find  a  rock,  like  the  white  marble  of 
Vermont,  which  the  examination  of  the  chemist  shows  to 
be  alJ  of  the  same  chemical  substance,  and  which  has 
throughout  the  same  characters  of  hardness,  density,  color, 


MINERALS  AND   MINERALOGY.  5 

and  so  on;  then  it  is  said  to  be  a  mineral  itself,  and  not, 
like  most  rocks,  a  mixture  of  a  variety  of  different  min- 
erals. 

These  different  kinds  of  substances,  then,  which  make 
up  the  rocky  material  of  the  earth  so  far  as  we  can 
study  it,  and  into  which  we  can  separate  the  seashore 
sand,  the  granite,  and  most  other  rocks,  are  called  MIN- 
ERALS. Each  one  has,  first  of  all,  a  definite  chemical 
composition,  wherever  it  is  found.  Moreover,  if  in  the 
form  of  a  crystal,  it  has  a  shape  of  its  own,  too,  by  which 
it  may  be  distinguished;  it  has  also  certain  characters 
of  hardness  and  density,  luster,  color,  transparency,  and 
others.  And  because  to  it  belong  all  these  different  char- 
acters, which  distinguish  it  from  other  kinds,  it  is  called  a 

MINERAL  SPECIES. 

It  is  the  work  of  the  mineralogist  to  study  these  min- 
erals; to  learn  all  the  different  kinds;  what  the  characters 
of  each  are;  how  they  are  classified  and  how  distinguished 
from  each  other;  how  they  occur  in  nature;  and  some- 
thing about  their  practical  uses. 

All  the  knowledge  which  the  many  mineralogists  have 
learned,  after  long  years  of  patient  observation  and  study, 
both  in  the  field  and  the  laboratory,  has  been  arranged  in 
systematic  form  and  makes  up  the  Science  of  Mineralogy. 

The  question  as  to  what  particular  minerals  go  together 
to  make  the  different  kinds  of  rocks,  how  these  are  formed, 
and  what  changes  of  position  or  of  character  they  have 
experienced— all  these  and  other  similar  questions  are  re- 
ferred to  the  geologist.  The  science  of  the  geologist,  or 
geology,  is  much  broader  than  mineralogy:  it  treats  of 


6  MINERALS,    AND    HOW   TO   STUDY   THEM. 

the  history  of  the  earth  and  all  the  changes  it  has  gone 
through;  the  different  kinds  of  rocks;  the  way  the  moun- 
tains have  been  built  up  from  them;  the  growth  and  de- 
velopment of  different  kinds  of  life  from  the  earliest  times 
down  to  the  present. 

It  was  stated  at  the  beginning  of  the  chapter  that  min- 
erals belong  to  the  MINERAL  KINGDOM;  but  it  is  important 
to  remember  that  all  substances  mineral  in  nature  are  not 
necessarily  called  minerals. 

The  mineralogist,  for  example,  usually  excludes  from 
his  cabinet  many  mineral  substances,  such  as  the  pearl  of 
the  oyster-shell  and  the  shell  itself,  the  lime  of  the  bones 
of  animals,  and  the  opal-like  form  of  silica  secreted  by 
the  growth  of  plants,  as  the  tabasheer  found  in  the  joints 
of  the  bamboo.  In  general  mineral  substances  such  as 
these,  which  are  formed  immediately  by  the  processes  of 
animal  or  vegetable  life,  are  not  called  minerals. 

Further,  the  mineralogist  does  not,  as  a  rule,  admit  among 
minerals  gases  like  the  oxygen  and  nitrogen  which  make 
up  the  air;  and  of  the  liquids  he  includes  only  the  metal 
mercury,  and  perhaps  also  water. 

The  many  beautiful  kinds  of  salts  made  by  the  chemist 
are  also  not  called  minerals.  The  rock  salt  or  sodium 
chloride  which  is  mined,  sometimes  in  fine  clear  cubi- 
cal blocks,  is  the  same  sodium  chloride  which,  as  the 
table-salt  of  every-day  life,  is  so  commonly  used.  But  the 
table-salt  obtained  from  evaporating  sea  -  water  or  the 
brines  of  salt-wells,  or  from  the  solution  of  crude  rock 
salt,  though  when  manufactured  it  may  be  formed  in  crys- 


MINERALS   AND    MINERALOGY. 

tals  as  fine  as  those  found  in  the  rocks,  is  not  called  a 
mineral,  because  not  made  by  nature  alone.  So,  too,  the 
fine  crystals  of  blue  vitriol,  or  copper  sulphate,  made  by 
the  chemist,  do  not  find  a  place  in  a  mineral  cabinet, 
though  the  much  less  fine  specimens  of  the  same  material 
found  in  some  of  the  Arizona  mines  do.  In  the  same 
way,  the  crystals  of  the  metals  and  of  many  interesting 
compounds  formed  in  the  metallurgical  process  of  making 
iron  or  lead  or  zinc  are  called  furnace-products,  and  not 
minerals.  These  substances,  however,  are  all  very  inter- 
esting, and  their  study  is  a  very  important  help  to  pure 
mineralogy.  In  recent  years  the  chemist  has  busied  him- 
self in  imitating,  so  far  as  he  can,  the  possible  processes  of 
nature,  and  thus  making  "artificial  minerals"  Recently 
the  diamond  has  been  formed  in  minute  crystals,  also  small 
but  fine  clear  rubies,  and  so,  too,  quartz,  feldspar,  mica, 
and  many  common  species. 

It  must  be  acknowledged,  however,  that  the  specimens 
thus  obtained  in  the  laboratory  are  in  most  cases  very 
minute  and  much  less  beautiful  than  those  of  nature;  for 
the  chemist  in  the  laboratory  has  only  a  limited  time  for 
his  experiments,  and  often  must  use  violent  means, — as 
the  great  heat  of  a  furnace. — while  nature  works  slowly 
and  gently 


MINERALS,   AND  HOW   TO  STUDY   THEM. 


CHAPTER  II. 

SOME  PRELIMINARY  HINTS  AS  TO   HOW  TO  STUDY 
MINERALS. 

A  MINERAL,  we  have  seen,  is  a  substance  formed  by 
nature  alone,  a  solid  with  one  or  two  exceptions,  and  one 
having  as  a  rule  a  definite  form  of  its  own  and  certain 
characters  of  hardness,  density,  luster,  color,  and  still 
others,  and,  most  important  of  all,  a  definite  chemical  com- 
position. The  first  group  of  characters,  having  to  do  with 
the  form  and  structure  and  so  on,  are  called  PHYSICAL 
CHARACTERS,  while  those  depending  directly  upon  the 
composition  are  called  CHEMICAL  CHARACTERS.  All  of 
these  will  be  described  in  some  detail  in  subsequent  chap- 
ters, but  it  is  necessary  first  to  gain  a  little  knowledge  as 
to  how  to  study  minerals,  where  the  object  is  to  learn  as 
much  as  possible  about  each  and  to  distinguish  one  kind 
from  another. 

The  mineralogist  must  first  of  all  use  his  eyes  and  other 
unaided  senses  in  studying  minerals;  in  other  words,  he 
must  gain  all  the  information  he  can  about  minerals  by 
looking  at  them  and  handling  them.  If  he  learns  to  do 
this  wisely,  he  will  be  surprised  to  find  how  keen  his 
senses  become  and  how  much  he  can  find  out.  But  as  he 
gains  in  experience  he  will  see  that  this  only  carries  him 
to  a  certain  point,  and  he  should  always  recognize  the  im- 


HINTS   ON   THE   STUDY   OF   MINERALS.  9 

portance  of  confirming  the  conclusions  reached  by  his  eye 
and  hand  by  more  positive  tests.  Often,  even  in  the  case 
of  the  commonest  species,  the  appearance  may  lead  one 
who  depends  upon  it  alone  quite  astray.  The  old  saying 
that  "  all  is  not  gold  that  glitters,"  and  the  names  ap- 
plied to  certain  common  minerals  of  "fool's  gold,"  "false 
galena,"  and  others  like  them,  express  the  result  of  experi- 
ence that  the  senses  unassisted  may  readily  be  deceived. 

The  trained  eye  of  the  mineralogist  will  show  him,  first 
of  all,  the  form  of  the  mineral,  as  to  whether  it  has  the 
regular  geometrical  shape  of  a  crystal  or  not,  or  is  simply 
granular,  fibrous,  and  so  on.  It  will  show  him  whether 
it  has  the  natural,  easy,  smooth  fracture  of  many  crystal- 
lized substances,  called  cleavage,  or  only  the  fracture  of 
ordinary  kinds.  It  will  tell  him,  too,  what  peculiarities  of 
luster  the  surface  of  a  mineral  presents,  depending  upon 
the  way  in  which  it  reflects  light,  whether  metallic, 
glassy,  greasy,  or  silky,  and  so  on;  also  what  the  color  is, 
whether  it  is  transparent  or  opaque,  and  many  other 
points. 

The  touch  will  show  whether  the  "  feel "  is  greasy,  as  is 
true  of  a  few  very  soft  minerals,  or  harsh,  as  are  the 
majority.  Again,  a  mass  in  the  hand  will  often  be  recog- 
nized at  once  as  heavy  or  light  as  compared  with  familiar 
substances  of  the  same  appearance.  Thus  the  common 
minerals  quartz,  feldspar,  calcite,  have  nearly  the  same 
density,  and  one  can  easily  become  so  accustomed  to  them 
that  a  piece  of  gypsum  seems  light  and  one  of  barite 
(heavy  spar)  seems  heavy.  So  a  piece  of  the  metal  alu- 
minium seems  very  light  because  we  instinctively 


10  MINERALS,   AND   HOW  TO   STUDY  THEM. 

it  with  the  other  much  denser  metals  which  we  are  ac- 
customed to  handle. 

The  taste  may  sometimes  tell,  for  instance,  that  rock 
salt  is  in  hand,  while  the  odor  is  occasionally  a  useful 
character,  as  the  clayey  odor  of  some  minerals  when 
breathed  upon. 

But  it  requires  some  education  and  experience  before 
the  senses  are  so  on  the  alert  that  all  the  characters  noted 
are  perceived  at  once  and  rightly  estimated;  this  is  what 
every  one  should  strive  for;  and  one  of  the  great  benefits  to 
be  derived  from  the  study  of  mineralogy  is  that  it  culti> 
vates  and  stimulates  the  powers  of  observation. 

When  the  senses  alone  stop,  simple  tests  to  aid  them 
come  in.  A  touch  upon  the  smooth  surface  of  a  mineral 
with  the  point  of  a  knife  serves  to  show  whether  it 
is  relatively  soft  or  hard.  The  color  of  the  powder  ob- 
tained by  scratching  with  the  knife  or  upon  a  plate  of 
rough  porcelain  or  ground  glass,  called  the  streak,  is  some- 
times quite  different  from  that  of  the  surface,  and  in  such 
cases  this  is  a  very  important  character. 

Then  come  more  careful  tests:  the  determination  of  the 
relative  density  or  specific  gravity;  the  use  of  the  blow- 
pipe, giving  the  comparative  degree  of  fusibility;  and  a 
number  of  simple  chemical  trials,  to  show  the  presence 
of  sulphur  or  arsenic,  silver,  lead  or  iron,  barium  or  stron- 
tium. Then  follow  still  other  tests,  till  we  come  to  the  re- 
fined methods  of  the  trained  mineralogist  with  his  beauti- 
ful goniometer  for  measuring  angles,  the  microscope  and 
optical  instruments,  the  accurate  chemical  analysis  and 
other  means  by  which  most  of  nature's  secrets  may  be 


HINTS   ON   THE   STUDY    OF   MINERALS.  11 

learned  and  the  characters  of  each   mineral   thoroughly 
studied. 


SOME    SUGGESTIONS     ABOUT     MAKING    A    COLLECTION    OF 
MINERALS. 

A  very  important  matter  in  the  study  of  minerals  is  the 
student's  own  collection;  for  every  one  who  desires  to 
really  learn  mineralogy  must  have  a  collection  of  his  own 
to  examine  and  experiment  upon.  It  is  very  desirable  that 
the  school  or  college  should  have  a  larger  cabinet  for  refer- 
ence and  study,  but  this  does  not  take  the  place  of  tfhe  in- 
dividual collection,  which  will  be  studied,  arranged,  labeled, 
and  handled  over  and  over  again  till  every  specimen  is  per- 
fectly familiar. 

Further,  the  student  should  obtain  his  specimens  as  far 
as  he  can  by  collecting  for  himself.  No  matter  if  he  lives  in 
a  region  that  does  not  seem  at  first  to  afford  very  much, 
he  can  certainly  find  something  that  is  worth  keeping 
until  he  obtains  better;  and  occasionally  he  will  have  the 
opportunity  to  make  trips  to  some  of  the  noted  localities, 
where  he  can  find  more  and  a  great  variety.  There  is 
nothing  more  delightfully  instructive  and  health-giving 
than  to  spend  a  day  in  the  open  air,  with  a  good  hammer 
in  hand,  a  bag  for  the  specimens,  and  plenty  of  soft  paper 
(and  perhaps  some  cotton)  to  wrap  them  up  in. 

The  hammer  should  be  of  hard  steel  that  will  not  chip 
on  the  edges;  it  may  weigh  from  a  pound  to  a  pound  and  a 
half,  and  the  face  should  be  square  or  slightly  oblong  and 
the  edges  sharp,  while  the  back  has  the  form  of  a 


12  MINERALS,    AND   HOW   TO   STUDY   THEM. 

wedge,  as  seen  in  Figure  1  (one-fourth  natural  size).  A 
cold-chisel  or  two,  for  working  into  cracks  or  crevices, 
will  often  be  found  useful;  also  a 
small  light  hammer  with  a  sharp  edge 
for  trimming  specimens.  This  will 
often  do  no  damage,  when  a  blow  from 
a  heavy  hammer  would  shatter  the 
specimen  and  destroy  it. 

Do  not  break  the  crystals  out  of  the 
rock,  as  a  rule.  A  detached  crystal 
of  garnet  is  interesting  when  quite 
perfect,  but  in  general  the  crystal  is 
most  interesting  and  instructive  when 
in  its  own  home.  The  seller  of  min- 
erals soon  discovers  this,  and  it  is  un- 
fortunately not  an  uncommon  trick 
at  some  localities — for  instance  in  the 
Alps— for  the  local  collector  working 
for  his  daily  bread  to  exercise  his 

ingenuity  in  mounting  a  loose  crystal 
J4  natural  size.  .  .     .          ,  .  ,      . 

in  a  mass  of  rock  in  which  it  never 

belonged,  thus  to  increase  the  value  of  the  specimen  and 
deceive  the  unwary  purchaser. 

The  student  is  not  advised  to  spend  a  great  deal  of 
money  in  buying  specimens,  particularly  at  any  one  time. 
Still  it  is  less  easy  to  collect  personally  now  than  it  was 
years  ago,  and  many  students  may  not  have  opportunity  to 
do  the  traveling  that  it  requires:  and  even  here  the  reward 
is  often  small,  unless  at  a  quarry  or  mine  where  work  is 
being  carried  on  all  the  time. 


HINTS   ON   THE   STUDY   OF   MINERALS.  13 

Hence  a  little  money  is  by  no  means  thrown  away  if 
judiciously  expended  from  time  to  time,  for  it  will  serve  to 
buy  a  few  small  characteristic  specimens  of  the  common 
species  and  pure  fragments  for  blowpipe  tests.  Fine 
specimens,  especially  of  the  rarer  species,  are  now  very 
expensive,  but  sufficiently  good  ones  of  the  minerals  it  is 
important  for  the  student  to  know  well  may  be  obtained, 
for  very  little  money.* 

It  is  better  to  collect  small  specimens  rather  than  large, 
as  far  as  possible,  such  as  will  go  in  a  little  paper  tray  2 
inches  square,  or  2  by  3  inches,  or  at  most  3x3.  These 
trays  are  inexpensive  and  are  very  useful  for  the  arrange- 
ment and  preservation  of  a  cabinet.  If  the  specimens  are 
placed  loose  in  a  drawer,  it  can  hardly  be  opened  a  few 
times  without  throwing  them  into  confusion,  and  sooner  or 
later  they  will  be  badly  injured.  The  sizes  mentioned  are 
the  most  useful,  though  3x4  inches  might  well  be  added. 
A  depth  of  half  an  inch  is  sufficient  for  the  tray,  but  the 
drawers,  if  possible,  should  not  be  less  than  2£  or  3  inches 
deep.  All  the  specimens  in  a  collection  should  be  care- 
fully labeled,  particularly  as  regards  the  locality. 

*A  list  of  the  most  important  minerals  is  given  at  the  close  of  the 
book,  and  those  most  useful  for  trial  with  the  blowpipe  are  there 
indicated. 


14  MINERALS,   AND   HOW  TO   STUDY   THEM. 


CHAPTER  III. 
THE   FOKMS  OF  CKYSTALS  AND  KINDS  OF  STRUCTURE. 

THE  principal  characters  of  minerals,  by  which  one 
species  is  distinguished  from  another,  have  been  briefly 
alluded  to  in  the  preceding  chapter.  It  is  now  necessary 
to  study  some  of  these  characters  more  fully. 

First  the  PHYSICAL  CHARACTERS  will  be  considered. 
These  include  the  form  and  structure,  the  cleavage,  frac- 
ture, hardness,  tenacity,  elasticity;  also  the  density;  fur- 
ther, the  color,  luster,  degree  of  transparency,  and  some 
few  others.  The  present  chapter  is  limited  to  a  discussion 
of  the  crystalline  form  and  the  structure  in  general. 

THE  GENERAL  CHARACTERS  OF  CRYSTALS. 

If  we  examine  the  specimens  of  the  different  mineral 
species  in  a  good  cabinet,  we  see  that  many  of  them  occur 
commonly  in  regular  solid  forms  with  smooth  faces,  which 
forms,  as  we  study  the  subject  further,  we  find  to  be  char- 
acteristic of  each  individual  species.  These  regular  forms 
are  called  crystals.  The  cubes  of  fluorite  (Fig.  2)  or 
galena,  the  six-sided  prisms  of  quartz  (Fig.  3),  the  twelve- 
sided,  twenty-four-sided,  or  even  more  complex  forms  of 
garnets,  are  common  examples  of  crystals.  Further,  even 
when  a  specimen  does  not  show  this  regular  external  form, 
there  is  usually  a  definite  crystalline  structure,  which  may 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    15 


be  shown  in  the  easy  fracture  called  cleavage,  as  that  of 
calcite,  or  which  may  be  indicated  in  other  ways,  as  we 
shall  soon  learn.  How  is  this  regularity  of  form  and 
structure  to  be  explained  ?  First,  we  will  speak  of  crystals. 
The  physicist,  as  the  result  of  his  studies  into  the  struc- 
ture of  different  kinds  of  matter,  has  concluded  that  every 


te,  Group  of  Cubic  Crystals. 


Quartz  Crystal. 


body  is  built  up  of  minute  particles  called  molecules, 
which  are  much  too  small  to  be  perceived  even  by  the 
strongest  microscope.  In  a  solid  body,  as  a  lump  of  iron, 
ice,  sulphur^  he  thinks  of  these  molecules  as  bound  to- 
gether by  a  strong  force  of  attraction,  called  cohesion,  so 
that  it  requires  a  hard  blow  or  great  pressure  to  change  its 
shape.  In  a  liquid  body  he  thinks  of  them  as  free  to  move 
or  roll  over  each  other,  so  that  the  liquid  takes  at  once  the 
shape  of  the  vessel  in  which  it  is  contained,  whatever  that 
may  be.  In  a  gas,  he  believes  that  the  molecules  are  sepa- 


16  MINERALS,    AND   HOW  TO   STUDY   THEM. 

rate  from  each  other,  a  long  distance  in  fact  compared 
with  their  size,  and  that  they  are  darting  about  very  r«ip- 
idly,  colliding  against  each  other  and  any  confining  surface. 
The  result  of  this  is  that  the  gas  at  once  fills  entirely  a 
vessel  into  which  it  is  introduced  and  presses  against  its 
sides;  the  pressure  being  simply  the  result  of  the  bombard- 
ment of  these  little  rifle-balls.  The  pressure  of  the 
external  air,  for  example,  is  shown  by  the  collapse  of  the 
cheeks  when  the  air  within  the  mouth  is  drawn  away. 

The  relation  between  these  minute  particles  or  molecules 
thus  explains  the  condition  of  a  body,  as  solid,  liquid,  or 
gaseous;  for  example,  the  distinction  between  ice,  water, 
and  steam. 

But  more  than  this:  When  a  liquid  turns  into  a  solid 
because  the  temperature  falls,  as  when  water  freezes,  or 
liquid  sulphur  or  molten  iron  hardens  on  cooling,  the  force 
of  cohesion  comes  into  play  to  bind  the  particles  together 
into  a  rigid  mass.  So,  also,  when  by  slow  evaporation  from 
a  jolution,  as  of  salt  or  alum  in  water,  the  dissolving  liquid 
is  removed,  the  substance  in  solution  also  passes  back  into 
the  solid  form  under  the  action  of  this  same  force  of  cohe- 
sion. Thus  the  solid  is  formed  from  the  liquid  by  the 
action  of  the  forces  acting  between  these  little  particles. 
Further,  if  the  molecules  are  all  of  one  kind,  as  in  a  given 
chemical  substance,  and  if  there  are  no  hindering  causes, 
these  molecules  will  build  themselves  up  after  some  regular 
pattern  and  the  external  result  is  the  geometrical  form, 
which  is  called  a  crystal.  It  is  somewhat  as  if  the  mole- 
cules were  little  building-stones,  built  up  into  a  solid 
structure  by  forces  acting  between  them  and  causing  them 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     17 

to  arrange  themselves  after  a  definite  manner  when  they 
are  free  to  do  so. 

This  regular  building  of  the  molecules,  which,  as  has 
just  been  shown,  may  take  place  from  a  liquid,  happens 
also,  even  more  perfectly,  when  a  solid  is  formed  direct 
from  a  gas.  Water  vapor  in  the  air,  if  cooled  down  suf- 
Sciently,  is  formed  into  the  solid  snow,  and  the  little  snow- 
crystals,  that  fall  silently  through  the  atmosphere  and 
which  we  may  catch  on  our  coat-sleeve  on  a  cold  winter 
day,  are  often  of  wonderful  regularity  and  beauty  of 
form.  The  figure  (4)  gives  some  of  the  many  forms  of 
snow-crystals  drawn  by  Scoresby  in  a  visit  to  the  Arctic 
4. 


Snow -crystals. 

many  years  ago.  So  too,  as  will  be  described  later,  if  a 
mineral  containing  arsenic  is  heated  in  a  glass  tube  open 
at  both  ends,  the  arsenic  driven  off,  uniting  with  the 
oxygen  of  the  air, forms  the  vapor  of  oxide  of  arsenic;  this 
is  condensed  a  little  higher  in  the  tube,  where  it  is  cooler, 
and  there  deposited  in  minute  spangling  octahedral  crys- 
tals (Fig.  6,  p.  23),  which  are  sometimes  quite  large  and 
very  perfect  in  form. 

It  is  not  always  easy  to  make  good  crystals,  whether 
starting  from  a  liquid  or  from  a  gas.  This  is  true  in  part 
because  we  cannot  give  the  time  required  for  the  perfect 
p/ocess,  in  part  because  there  are  other  hindering  con- 


18  MINERALS,  AND   HOW   TO   STUDY   THEM. 

ditions.  But  sometimes  we  can  succeed  well,  and  the 
growth  of  an  octahedral  crystal  of  alum  in  a  strong 
solution  can  be  watched  from  day  to  day  and  a  large  and 
fine  crystal  may  be  the  reward  of  our  skill  and  patience. 

In  nature's  laboratory  the  conditions  are  more  favorable, 
particularly  because  there  is  never  any  limit  of  time,  and 
the  many  beautiful  and  complex  crystals  of  minerals  with 
brilliant  faces  show  the  result.  Even  here,  however,  the 
building  process  often  cannot  go  on  freely,  and  imperfect 
crystals,  or  perhaps  a  mass  with  only  a  confused  crystal- 
line structure  and  without  distinct  external  form,  may  be 
all  that  is  produced. 

The  quartz,  feldspar,  and  mica  in  the  rock  called  granite 
have  usually  formed  together  in  such  a  way  that  neither 
one  has  had  an  opportunity  to  build  itself  up  into  perfect 
crystals,  and  yet  the  student  who  understands  the  optical 
study  of  thin  sections  of  a  rock  in  polarized  light  can 
prove  that  each  grain,  formless  though  it  may  be  externally, 
has  all  the  internal  molecular  structure  of  the  crystal.  In 
a  cavity  in  the  granite  we  are  not  surprised  to  find  crystals 
of  quartz  and  feldspar,  perhaps  also  of  mica,  as  the  cavity 
here  means  that  each  has  had  an  opportunity  to  exercise 
its  tendency  to  build  itself  regularly  with  something  of  the 
freedom  which  a  perfect  crystal  requires. 

Another  familiar  example  of  crystallization  is  given  by 
the  ice  covering  a  pond,  which  is  as  truly  crystalline  in 
structure  as  the  perfect  snow-crystal;  but  here  there  are  no 
crystals,  and  it  is  easy  to  understand  why.  The  slow  dis- 
section of  the  mass,  however,  under  the  melting  action  of 
the  sun  reveals  something  of  the  regularity  in  the  molec 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     19 

ular  building,  and  the  same  thing  is  proved  by  an  exami- 
nation in  polarized  light.  Sometimes  in  the  freezing  of 
a  little  pool  of  water  on  a  sidewalk  the  formation  of  the 
slender  crystalline  ribs  of  ice  may  be  watched  as  they 
shoot  out,  forming  a  framework  which  may  soon  lose  its 
distinctness  as  the  entire  surface  is  frozen  over. 

We  learn,  then,  that  a  crystal  is  the  regular  solid  form 
which  a  chemical  substance  takes  when  it  passes  into  the 
solid  state  from  that  of  either  a  liquid  or  a  gas,  if  under 
such  conditions  that  the  molecules  are  quite  free  to  ar- 
range themselves  according  to  the  direction  of  the  at- 
tractive forces  acting  between  them. 

The  crystal  is,  therefore,  the  outward  expression  of  the 
structure  in  the  arrangement  of  these  molecules,  and  itt 
form  is  for  this  reason  the  most  important  of  all  the  physi- 
cal characters  of  a  given  species  and  the  one  which  in  gen- 
eral most  definitely  distinguishes  it  from  others. 

It  is  interesting  to  note  that  a  small  crystal  is  just  as  per- 
fect and  complete  an  individual  as  a  similar  one  of  great 
size;  there  is  among  the  crystals  of  a  given  species  no  such 
connection  between  size,  on  the  one  hand,  and  age  and 
maturity,  on  the  other,  that  belongs  to  the  individuals  of  a 
species  in  the  animal  and  vegetable  kingdoms.  Some 
crystals  are  so  minute  as  to  be  almost  microscopic;  others 
may  be  of  enormous  size,  as  the  gigantic  quartz  crystals 
occasionally  found  in  the  Alps,  or  the  equally  large  beryl 
crystals  from  New  Hampshire.  A  cave  opened  a  few  years 
ago  at  Macomb,  New  York,  contained  15  tons  of  great 
cubic  crystals  of  fluorite;  another  cave  in  Wayne  County, 
Utah,  contained  a  great  number  of  enormous  crystals  of 


MINERALS,  AND    HOW   TO   STUDY   THEM. 

gypsum,  some  of  them  three  feet  or  more  in  length.  But 
the  very  small  crystals  and  the  like  ones  of  enormous  size 
are  not  essentially  different  except  in  this  comparatively 
unimportant  respect  of  magnitude. 

And  yet  there  are  many  interesting  points  of  resem- 
blance between  crystals  and  living  plants.  Crystals  grow 
as  well  as  plants,  and  under  favorable  conditions  so 
rapidly  that  the  increase  in  size  may  be  watched  not  only 
from  day  to  day,  but  from  hour  to  hour,  or  even  from 
minute  to  minute.  The  complex  forms  that  are  built  up 
especially  in  such  cases  of  rapid  growth  are  often  wonder- 
fully plantlike  in  aspect.  This  is  true,  as  every  one  has 
noticed,  of  the  delicate  frost-figures  which  form  so  quickly 
upon  a  window-pane  or  a  paving-stone  in  winter;  also,  in 
other  more  permanent  cases,  the  arborescent  or  dendritic 
forms  of  native  gold  or  silver  or  copper  are  good  examples 
of  the  same  fact.  The  terms  used  in  describing  them  are 
indeed  given  because  of  their  resemblance  to  forms  of 
vegetation.  Furthermore,  as  a  wounded  plant  tends  to 
heal  itself  when,  for  example,  a  branch  has  been  broken 
off,  or  as  a  slip  or  graft  tends  to  develop  a  full  individual; 
so,  too,  a  broken  crystal  may  be  more  or  less  healed,  but  in 
the  last  case  the  material  which  repairs  the  injury  must  be 
supplied  from  an  outside  source.  Thus  the  silica  to  mend 
a  broken  quartz  crystal  must  come  from  a  foreign  solution, 
and  the  crystal  itself  only  directs  the  way  in  which  the 
molecules  of  the  solution  are  laid  down;  it  is  interesting, 
however,  that  the  growth  takes  place  more  readily  on  a 
surface  of  fracture  than  on  a  natural  crystalline  face.  In 
this  way  the  grains  of  quartz  in  a  sandstone,  formless  be- 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    21 

cause  only  fractured  fragments,  often  tend  to  build  them- 
selves up  into  complete  crystals. 

Still  again,  although  a  crystal  never  has  an  old  age  in 
the  sense  that  this  is  true  of  a  plant  or  an  animal,  it  is 
nevertheless  a  fact  that  many  crystals  tend  to  change  or 
decay  as  time  goes  on,  if  subjected,  for  example,  to  the 
corroding  effects  of  some  foreign  solution. 

Even  the  beautiful  gems,  such  as  the  sapphire,  emerald, 
topaz,  garnet,  hard  and  comparatively  insoluble  as  they 
are,  have  this  liability  to  undergo  what  is  called  chemical 
decomposition,  with  the  loss  of  all  their  beauty  and  a  total 
change  of  chemical  substance.  This  is  spoken  of  again  in 
a  later  part  of  the  chapter,  where  pseud omorphs  are  de- 
scribed, but  it  is  worth  noting  here  because  somewhat 
analogous  to  the  change  that  old  age  brings  to  a  living 
organism. 

THE  SYSTEMS  OF  CRYSTALLIZATION. 

The  forms  of  crystals  are  so  varied  and  the  difficulties 
in  studying  them  minutely  so  great  that  we  shall  only 
attempt  here  to  learn  some  of  their  simplest  kinds. 

In  the  first  place,  it  is  important  to  understand  that  it 
can  be  shown  that  all  crystals  belong  to  one  of  six  classes, 
or  systems,  which  are  named  as  follows:  I,  ISOMETRIC; 
II,  TETRAGONAL;  III,  HEXAGONAL;  IV,  ORTHORHOMBIC; 
V,  MONOCLINIC;  VI,  TRICLINIC. 

The  characters  of  each  system  and  the  relations  between 
them  will  be  briefly  mentioned  after  the  chief  forms  in 
each  have  been  described. 


22  MINERALS,  AND   HOW  TO   STUDY  THEM. 


I.  Isometric  System. 

The  principal  forms  of  the  Isometric  System  are  the 
cube,  octahedron,  dodecahedron,  the  two  trisoctahedrons, 
the  tetrahexahedron,  and  the  hexoctahedron. 

Cube. — The  cube  has  six  equal  faces,  each  one  of  which 
is  a  square,  and  the  angle  between  any  two  faces  is  a  right 
angle,  or  90°.  It  is  shown  in  Fig.  5.  Galena  and  fluorite 
often  occur  in  cubes. 

Octahedron. — A  regular  octahedron  (Fig.  6)  has  eight 
like  faces,*  each  a  triangle  with  equal  sides  and  three 
equal  angles  (each  60°);  the  angle  between  any  two  adja- 
cent faces  is  109°  28'.  Magnetite  is  often  in  octahedrons. 

Dodecahedron. — The  rhombic  dodecahedron  (Fig.  7)  has 
twelve  equal  faces,*  each  of  which  is  a  rhomb  with  plane 
angles  of  60°  and  120°,  while  the  angle  between  two  adja- 
cent faces  is  120^ 7TEis  is  a  common  form  with  garnet. 

These  forms  may  occur  together  on  the  same  crystal. 
Thus  crystals  of  galena  often  show  the  cube  and  octa- 
hedron together.  Fig.  8  is  generally  described  as  a  cube 
modified  by  an  octahedron,  and  Fig.  9  as  an  octahedron 
modified  by  the  cube.  If  a  cube  is  cut  out  of  a  block  and 
the  solid  angles  sliced  away  carefully,  the  new  surface 
making  equal  angles  with  the  three  cubic  faces,  the  result 
is  to  give  finally  an  octahedron.  It  is  seen  that  the  octa- 
hedral faces  are  little  triangles  on  the  solid  angles  of  the 

*  Octahedron  is  named  from  the  Greek  otcroi,  eight,  and  eSpa, 
face,  or  the  eightfaced  solid.  Dodecahedron  is  similarly  named 
from  SooSetca,  twelve,  and  t=8 pa,  fact. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.  23 


cube  and  equally  inclined  to  the  three  cubic  faces.  On 
the  other  hand,  the  cubic  faces  are  small  squares  on  the 
six  solid  angles  of  the  octahedron.  The  angle  between 
two  adjacent  faces  of  a  cube  and  an  octahedron  is  125°  16'. 
Figures  10,  11  show  the  cube  and  dodecahedron  -to- 


gether, and  Fig.  12   the   octahedron  and  dodecahedron. 

Both  the  cube  and  octahedron  have  twelve  similar  edges, 

and  these  are  cut  off  equally,  or  truncated,  by  the  twelve 

faces  of  the  dodecahedron.     In  Fig.  13  we 

have  again  a  form  (not  shaded)  resulting 

from  the  combination  of  the  faces  of  the 

cube  (a),  octahedron  (o),  and  dodecahedron 

((/).     The  angle  between  adjacent  faces  of 

the   cube   and   dodecahedron  is  135°;  b<>- 

tween  those  of  the  octahedron  and  dodecahedron  it  is 

144°  44'. 

Trapezohedron. — A  trapezohedron  has  twenty-four  equal 
faces,  each  a  four-sided  figure  or  tranezoid.     It  is  shown 


MINERALS,  AND   HOW  TO   STUDY   THEM. 


in  Fig.  14,  which  is  a  common  form  with  garnet.  There 
may  be  a  large  number  of  different  trapezohedrons,  all 
having  the  same  general  form  but  differing  in  the  angles 
between  the  faces.  A  similar  remark  may  be  made  about 
each  of  the  other  type-forms  of  this  system  yet  to  be  de- 


scribed. It  requires  much  more  study  than  is  possible  for 
the  beginner  to  leani  how  these  forms  are  mathematically 
distinguished  from  one  another. 

Figures  16  to  19  show  combinations  of  the  trapezohe^ 


16. 


17. 


dron  (n  or  m)  wuli  the  cube  (a),  octahedron  (o),  and 
dodecahedron  (d).  The  last  two  are  common  forms  with 
garnet. 

The  trapezohedron  is  also  called  a  tetragonal  trisocta- 
hedron  because  its  form  suggests  an  octahedron  in  which 
three  faces  take  the  place  of  a  single  octahedral  face,  each 
of  them  being  a  four-sided  figure  or  tetragon. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     25 

There  is  also  another  trisoctahedron,  called  a  trigonal 
trisoctahedron,  shown  in  Fig.  15,  which  has,  again,  twenty- 
four  faces,  three  of  these  also  corresponding 
to  an  octahedral  face;  but  each  is  a  three- 
sided  figure  (trigon)  or  an  isosceles  triangle. 
This  form  does  not  often  occur  alone, 
but  may  be  seen  on  complex  crystals  of 
galena.  Fig.  20  shows  a  figure  of  galena 
with  the  cube  (a),  octahedron  (o),  dodeca- 
hedron (d)',  also  two  different  trigonal  trisoctahedrons, 
lettered  p  and  u. 

Tetrahexaliedron. —  A  tetrahexahedron    (Fig.   21)   is  a 

twenty-four-faced  solid,*  each  face  an  isosceles  triangle 

and  four  together  having  the  same  position  as  the  face  of  a 

cube.    Fig.  22  shows  a  combination  of  the  cube  and  a 

21.  22. 


tetrahexahedron ;  the  latter  is  said  to  bevel  the  edges  of  the 
cube  because  the  two  planes  are  equally  inclined  to  the 
two  adjacent  cubic  faces. 

Hexoctahedron. — A  hexoctahedron  (Fig.  23)  is  a  forty- 
eight-faced  solid;  each  face  is  a  scalene  triangle,  and  six 
faces  have  the  same  general  position  as  a  face  of  an  octa- 
hedron, f 

*  Named  from  rerpa,  four,  e£,  six,  and  edpa.  face. 
{  Named  from  e?,  six,oKroJ,  eight,  aud  ed pa,  fact. 


26  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Fig.  24  shows  a  combination  of  the  cube  (a)  with  the 
hexoctahedron;  it  is  a  common  form  with  fluorite.  Fig. 
25  is  a  common  garnet  form,  the  dodecahedron  (d)  with 
the  hexoctahedron,  the  latter  beveling  the  edges  of  the 
former. 

Many  of  the  figures  thus  far  given  and  some  of  those 
which  follow  are  shaded,  so  as  to  appear  solid  to  the  stu- 
dent learning  about  crystals  for  the  first  time.  It  is  ob- 


24. 


vious,  however,  that  when  the  form  is  complex  that  the 
shading  is  impossible,  and  for  the  experienced  crystallog- 
rapher  it  is  quite  unnecessary;  hence  these  more  complex 
figures  are  shown  in  line  only.  Through  the  rest  of  the 
book  these  line  figures  will  be  freely  used.  The  student 
will  soon  find  that  they  appear  as  solid  to  him  as  the 
others.* 

All  the  forms  that  have  been  mentioned  belong  to  what 
is  called  the  ISOMETRIC  SYSTEM,  in  which  the  crystallog- 
rapher  refers  the  planes  to  three  equal  axes  at  right  angles 

*  It  will  be  a  great  help  to  the  student  if  he  has  a  few  models  to 
handle.  These  are  made  in  great  perfection  in  wood  and  are  not 
very  expensive.  The  student  may  also  try  to  cut  them  for  himself 
out  of  soft  wood  or  plaster-of-paris;  even  a  potato  can  be  employed 
for  temporary  use. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     27 


to  each  other.     The  position  of  the  axes  passing  through 
the  centers  of  the  crystals  is  shown  in  Figs.  26,  27,  28. 


It  will  be  seen  to  be  true  of  all  these,  as  it  is  of  all  their 
combinations,  that  the  arrangement  of  the  faces  is  the 
same  about  each  one  of  the  six  cubic  faces,  or  in  other 
words  about  the  ends  of  the  three  axes. 

Another  way  of  stating  this  is  to  say  that  all  these 
isometric  crystals  have  three  equal  planes  of  symmetry  at 
right  angles  to  each  other.*  These  three  equal  planes  of 
symmetry  are  planes  parallel  to  the  cubic  faces  and  have  a 
corresponding  position  in  the  other  simple  crystals  or  com- 
binations of  them.  Each  plane  of  symmetry  divides  the 
ideal  crystal  into  two  symmetrical  halves,  and  here  the 
three  sets  of  halves  made  by  the  three  planes  parallel  to 
each  pair  of  cubic  faces  are  identical;  hence  the  planes  of 
symmetry  are  said  to  be  equal.  The  axes  are  the  lines  in 
which  these  three  planes  of  symmetry  intersect  each  other. 


*  A  plane  of  symmetry  is  a  plane  which  divides  the  solid  into 
equal  halves  such  that  if  one  half  is  placed  against  a  mirror  the  re- 
flection completes  the  form.  This  is  one  form  of  the  geometrical 
definition  applying  to  an  ideal  crystal ;  it  will  be  explained  later 
(p  50)  how  this  must  be  broadened  to  cover  the  crystal  lographic 
symmetry  of  actual  crystals. 


28 


MINERALS,  AND   HOW  TO   STUDY  THEM. 


A  cube,  as  well  as  the  other  isometric  forms  mentioned, 
has  also  six  other  planes  of  symmetry  passing  diagonally 
through  the  opposite  edges,  and  hence  parallel  to  each 
pair  of  the  dodecahedral  faces. 

29.  30. 


Fig.  29,  of  cuprite,  and  Fig.  30,  of  the  rare  species  micro- 
lite,  both  drawn  on  a  larger  scale,  are  added  to  show 
some  rather  complex  combinations  of  isometric  forms.  In 
Fig.  29  the  cube  (a)  and  the  dodecahedron  (d)  predomi- 
nate; the  faces  of  the  octahedron  (o)  are  small;  n  and  /? 
are  faces  of  two  different  trapezohedrons.  In  Fig.  30  the 
octahedron  (0)  predominates  and  then  the  cube  (a),  while 
the  dodecahedron  (d)  is  subordinate;  the  faces  m  belong 
to  a  trapezohedron,  and  p  to  a  trigonal  trisoctahedron. 

There  are  also  several  other  forms  belonging  to  the 
Isometric  System,  but  which  are  described  as  half-forms, 
or  forms  in  which  only  half  of  the  faces  in  the  correspond- 
ing whole  form  are  present.  To  them  the  rules  of  sym- 
metry do  not  apply,  but  their  faces  are  also  referred  to 
three  equal  axes  at  right  angles  to  each  other.  The  most 
important  of  these  half-forms  are  the  tetrahedron  and 
pyritohedron. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     29 


Tetrahedron.— The  tetrahedron  (Fig.  31)  is  a  form  with 
four  equal  triangular  faces,  each  of  them  an  equilateral 
triangle.  It  is  considered  in  Crystallography  as  the  half- 
form  of  the  octahedron,  since  half  the  faces  of  the  octa- 
hedron, taken  every  other  one,  will  if  extended  form  a 

31. 


tetrahedron.  Perhaps  a  study  of  Fig.  32  will  make  this 
clearer.  The  angle  between  two  adjacent  faces  of  a  te- 
trahedron is  70°  32'. 

Fig.  33  shows  a  combination  of  a  cube  (a)  with  the 
four  faces  of  a  tetrahedron  (0).  It  is  seen  that  the  planes 
are  present  only  on  the  alternate  angles  of  the  cube.  In 

35. 


Fig.  34,  a  combination  of  a  cube  and  a  tetrahedron,  the 
latter  predominates.  Fig.  35  shows  a  combination  of  the 
tetrahedron  before  figured  (o)  with  another  similar  form 
(lettered  o,)  made  up  of  the  four  remaining  faces  of  the 
octahedron.  It  might  be  asked  why  this  form  cannot  be 
regarded  as  an  octahedron  in  which  four  faces  are  ac« 


30 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


cidentally  larger  (compare  remarks  on  p.  49)  than  the 
others;  but  this  is  impossible,  for  it  can  be  proved,  per- 
haps at  once  by  difference  of  luster,  that  the  eight  faces 
are  not  all  alike,  but  only  four  and  four.  This,  however, 
is  a  somewhat  difficult  subject  for  a  beginner. 

Pyritohedron. — The  pyritohedron  (Fig.  36)  is  a  twelve- 
sided  solid,  or  dodecahedron,  each  face  of  which  is  a  penta- 
gon, but  not  here  as  with  the  pentagonal 
dodecahedron  of  geometry  a  regular  pen- 
tagon. In  crystallography  the  name 
dodecahedron  is  usually  given  only  to  the 
rhombic  dodecahedron  described  above 
(Fig.  7),  and  this  form,  the  pyritohedron, 
takes  its  name  from  the  species  pyrite  or  iron  pyrites, 
because  of  common  occurrence  with  it. 

The  pyritohedron  is  the  half-form  of  the  tetrahexahe- 
dron.  If  in  combination  with  the  cube,  the  solids  in 
Figs.  37,  38  result;  Fig.  39  is  a  combination  of  an 


octahedron  and  pyritohedron.  There  are  also  other  half- 
forms  in  the  Isometric  System,  thus  of  the  two  trisoctahe- 
drons  and  the  hexoctahedron;  but  they  are  not  very  com 
mon  and  will  not  be  described  here. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     31 

II.   Tetragonal  System. 

The  chief  forms  of  the  Tetragonal  System  are  the  two 
square  prisms  and  pyramids  and  the  eight-sided  prism 
and  double  eight-sided  pyramid. 

40.  41. 


IJ 


Square  Prism  and  Pyramid. — One  of  the  square  prisms 
is  shown  in  Fig.  40  and  the  square  pyramid  corresponding 
to  it  in  Fig.  41,  while  Fig.  42  is  a  combination  of  the 
two  forms. 

The  square  prism  has,  like  the  cube,  angles  of  90°  be- 
tween the  faces,  but  it  differs  from  the  cube  because  the 
four  vertical  faces  are  not  like  the  two  end  faces,  or  basal 
planes  as  they  are  called.  This  is  often  shown  in  a  crystal 
by  the  difference  in  the  smoothness  of  the  two  kinds  of 
faces;  or  there  may  be  easy  fracture,  or  cleavage,  parallel 
to  one  set  of  planes  and  not  to  the  other. 

The  square  pyramid  looks  somewhat  like  a  regular  octa- 
hedron, but  here  the  faces  are  isosceles  triangles  (not 
equilateral)  and  the  angle  between  two  faces  over  a  hori- 
zontal edge  differs  from  that  over  one  of  the  vertical 
edges— in  fact,  either  angle  is  characteristic  of  a  given 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


species  and  differs  from  one  species  to  another.  There 
may  be  a  great  many  square  pyramids  of  the  same  type  as 
this  but  differing  in  their  angles  and  consequently  flatter 
or  sharper  at  the  extremity. 

Fig.  43  shows  an  acute  square  pyramid,  p,  while  Fig.  44 

44, 


represents  another  crystal  of  the  same  species  (octahe- 
drite)  in  which  this  pyramid  p  is  present  but  with  it 
three  others,  z,  i,  v,  each  flatter  or  more  obtuse  at  the 
summit  than  the  others. 

There  is  also  another  square  prism  and  another  square 


45. 


46. 


pyramid  diagonal  to  the  set  just  described;  they  are 
shown  in  Figs.  45  and  46.  Fig.  47  shows  these  two  forms 
together.  Taken  alone  these  two  forms  cannot  be  dis- 
tinguished from  the  other  two  shown  in  Figs.  40,  41,  but 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     33 


49. 


if  they  occur  together  thfc  distinction  is  obvious.  Thus 
Fig.  48  shows  the  two  prisms  on  the  same  crystal,  the  faces 
of  one  truncating  the  edges  of  the  other. 

Figs.  49  and  50  show  the  same  two  prisms  in  skeleton 
lines  with  the  axes  represented 
inside;  it  is  seen  that  in  the 
prism  (Fig.  49)  first  mentioned 
— often  called  the  unit  prism — 
the  horizontal  axes  join  the 
middle  points  of  the  opposite 
edges,  while  in  Fig.  50  they  join  the  centers  of  the  oppo- 
site faces.  The  latter  form  is  often  called  the  diametral 
prism,  because  the  faces  are  parallel  to  the  axes  or  diam- 
eters. 

In  Figs.  51  and  52  the  two  pyramids  are  again  shown, 

51. 


and  here  the  position  of  the  axes  should  also  be  noted. 
Fig.  51  is  often  called  a  unit  pyramid  or  one  of  the  unit 
series;  Fig.  52  a  pyramid  of  the  diametral  series. 

In  Figs.  53,  54  the  combinations  of  each  square  prism 
with  the  pyramid  of  the  diagonal  set  are  shown.  Fig.  53 
resembles  a  cube  modified  by  an  octahedron  (Fig.  8,  p.  23), 
but  it  differs  from  it  in  that  the  faces  lettered  p,  while  they 
make  equal  angles  with  the  two  adjacent  faces  a,  make 


34 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


different  angles  with  the  basal  plane  or  base  c.     The  same 

statement  could  be  made  in  regard  to  the  form  of  Fig.  54. 

55  There  is  also  an  eight-sided  prism   made 

up  of  eight  like  faces,  and  it  is  shown  on  the 

Jjf^         complex  crystal  represented  in  Fig   59;   its 

/•>'  ^^    faces  are  lettered  h.     Further,  there  is  also  a 

j®jjgllB'     double  eight-sided  pyramid,  as  shown  in  Fig. 

\10  55.     This  is  often  called  a  zirconoid,  because 

common  with  the  species  zircon. 

In  Figs.  56,  57,  58,  representations  of  crystals  of  zircon, 
the  faces  x  (in  part  lettered)  belong  to  a  zirconoid  or 


57. 


88. 


double  eight-sided  pyramid.  The  same  is  true  of  the  faces 
lettered  z  in  the  figures  59,  60. 

Fig.  59  represents  a  complex  crystal  of  wernerite,  and 
Fig.  60  is  a  map  of  the  top  of  the  same  crystal,  or  a  basal 
section,  as  it  is  called.  Note  the  prism  and  pyramid  of 
the  unit  series,  m  and  r;  the  prism  and  pyramid  of  the 
diametral  series,  a,  e;  also  the  eight-sided  prism  h  and  the 
double  eight-sided  pyramid  z  already  referred  to. 

All  of  the  forms  that  have  been  described  belong  to 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    35 


what  is  called  the  TETRAGONAL  SYSTEM,  in  which  the 
planes  are  referred  to  three  axes  at  right  angles,  of  which 
the  two  in  the  horizontal  plane  are  equal,  the  third  or  ver- 
tical axis  is  longer  or  shorter. 

It  will  be  seen  by  examining  the  figures,  especially  Figs. 
58,  59,  60,  that  the  grouping  of  faces  is  the  same  about  the 
faces  lettered  a,  but  different  from  them  about  the  face  c, 
the  basal  plane.  It  will  be  seen,  too,  that  about  c  the 
faces  of  the  same  kind  are  all  arranged  in  fours  or  eights. 

In  other  words,  all  these  tetragonal  crystals  have  a  pair 

60. 

a 

^\m' 


of  equal  planes  of  symmetry  parallel  to  the  faces  a  and  at 
right  angles  to  each  other.  There  is  also  another  pair  at 
right  angles  to  each  other  parallel  to  the  faces  m.  All 
these  four  planes  meet  in  a  common  vertical  line,  which  is 
called  the  vertical  axis. 

There  is,  finally,  a  fifth  plane  of  symmetry  parallel  to 
the  top  and  bottom  of  the  crystal,  or  the  basal  plane  c,  and 
hence  at  right  angles  to  this  vertical  axis,  but  it  differs 
from  either  of  the  other  pairs. 

In  the  Tetragonal  System  there  are  also  some  half- 
forms  but  only  one  will  be  specially  described. 


36  MINERALS,  AND    HOW   TO   STUDY   THEM. 

Sphenoid. — The  sphenoid  (Fig.  61)  is  a  four- faced  solid 
61.  looking  like  a  tetrahedron,  but  differing  from 
it  since  the  faces  are  isosceles  (not  equilat- 
eral) triangles.  It  is  described  as  the  half- 
form  of  the  square  pyramid  shown  in  Figs. 
41  and  51. 

III.  Hexagonal  System. 

The  chief  forms  of  the  Hexagonal  System  are  the  two 
six-sided  prisms,  the  two  double  six-sided  pyramids,  and 
the  twelve-sided  prism  and  double  twelve-sided  pyramid. 

These  will  be  briefly  described  first,  and  then  the  char- 
acteristic forms  of  the  Rhombohedral  part  of  the  Hexago- 
nal System  will  be  mentioned. 

Hexagonal  Prism  and  Pyramid. — The  hexagonal  prism 
and  pyramid  are  shown  in  Figs.  62  and  63,  while  Fig.  64 
gives  a  combination  of  the  two.  The  angles  of  the  hexago- 
nal prism  are  exactly  120°,  and  the  terminal  face  or  basal 
plane  is  a  regular  hexagon.  The  faces  of  the  hexagonal 

62. 

-    " "       .  ,    , 


pyramid  are  isosceles  triangles,  differing  in  angle  accord- 
ing to  whether  the  pyramid   is   more   obtuse   or  acute, 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    37 


These  angles,  and  the  others  that  depend  upon  them,  are 
characteristic  for  a  given  species. 

There  is  also  another  hexagonal  prism  and  pyramid  di- 
agonal to  the  others  and  looking  much  like  them.  These 
two  sets  correspond  to  the  two  square  prisms  and  pyra- 

65. 


;m 


raids  of  the  Tetragonal  System.  Compare  Fig.  65  of  the 
unit  prism  and  Fig.  66  of  the  second  or  diagonal  prism 
and  note  the  position  of  the  axes  shown  in  each;  also  the 
position  of  the  axes  in  Fig.  67  of  the  unit  hexagonal  pyra- 
mid. Fig.  68  shows  the  combination  of  the  unit  hexago- 
nal prism  and  pyramid  with  the  basal  plane.  Fig.  69,  of 
a  crystal  of  beryl,  shows  a  combination  of  the  unit  prism 
and  pyramid,  m  and  p\  the  diagonal  prism  and  pyramid, 
a  and  s;  also  the  basal  plane  c. 

There  is  also  a  prism  bounded  by  twelve  similar  faces, 
and  a  double  pyramid  bounded  above  and  below  by  twelve 
triangular  faces.  This  double  twelve-sided  pyramid  is 
often  called  a  berylloid,  because  common  with  crystals  of 
beryl.  Two  berylloids  are  shown  in  Figs.  70,  71;  the 
faces  are  lettered  (in  part)  n  and  v  respectively.  Fig.  71 
is  an  enlarged  map,  or  basal  section,  of  the  top  of  a  crystal 
much  like  that  of  Fig.  70.  Note,  also,  on  Fig.  71  the  hex- 


38 


MINERALS,  AND   HOW  TO   STUDY   THEM. 


agonal  prism,  m,  and  pyramids,  u  and  p,  of  the  unit  series; 
the  prism,  a,  and  pyramid,  s,  of  the  diagonal  series. 
These  hexagonal  forms  belong  to  the  HEXAGONAL  SYS- 


69. 


70. 


71. 


TEM,  where  the  planes  are  referred  to  the  four  axes  shown 
in  Figs.  65,  66,  67,  three  in  a  horizontal  plane  equal  and 
cutting  each  other  at  angles  of  60°,  and  the  fourth  vertical 
axis  either  longer  or  shorter. 

It  will  be  seen  that  the  faces  are  arranged  in  the  same 
way  about  each  face  a  and  hence  about  each  end  of  the 
horizontal  or  lateral  axes,  but  differently  about  the  basal 
face  c,  that  is,  at  the  extremity  of  the  vertical  axes. 
The  faces  are  arranged  about  the  face  c  in  sixes  or  in 
twelves  instead  of  fours  and  eights  as  in  the  Tetragonal 
System. 

All  the  hexagonal  forms  have  three  equal  planes  of  sym- 
metry making  angles  of  60°  with  each  other  parallel  to  the 
faces  m;  also  another  set  of  three,  diagonal  to  the  others 
and  parallel  to  the  faces  a;  and  a  seventh  plane  parallel 
to  the  top  or  base  of  the  crystal. 

There  are  several  half-forms  in  the  Hexagonal  System, 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     39 

but  the  only  ones  that  will  be  described  here  are  those 
of  the  Rhombohedral  System. 

The  RHOMBOHEDRAL  SYSTEM  is  generally  treated  as  a 
branch  of  the  Hexagonal  System.  In  the  forms  belonging 
to  it  the  faces  are  in  threes  about  the  extremities  of  the 
vertical  axis  c,  and  there  are  only  three  vertical  planes  of 
symmetry  making  angles  of  60°  with  each  other  inter- 
secting in  this  axis.  The  important  forms  are  the  rhom- 
bohedron  and  the  scalenohedron. 

Rhombohedi'on. — The  rhombohedron  is  a  six-sided  solid, 
each  face  of  which  is  a  rhomb;  it  is  shown  in  Figs,  72,  73, 
74.  There  may  be  a  great  many  rhombohedrons,  as  shown 

72.  73. 


here,  differing  in  angle  and  consequently  more  or  less 
obtuse  or  acute.  The  rhombohedron  looks  somewhat 
like  a  cube  if  the  cube  is  placed  with  the  line  joining  two 
opposite  angles  vertical;  in  fact,  the  cube  comes  between 
the  obtuse  and  acute  rhombohedrons,  having  an  angle  of 
just  90°. 

The  rhombohedron  may  be  regarded  as  a  half-form  of 
the  hexagonal  pyramid,  but  this  subject  is  a  rather  diffi- 
cult one  and  cannot  be  followed  up  here. 

Scalenohedron. — The  scalenohedron  (Fig.  75)  is  a  twelve- 
sided  solid,  looking  a  little  like  a  double  six-sided  pyramid, 


40 


MINERALS,   AND   HOW   TO   STUDY   THEM. 


but  the  faces  are  scalene  triangles  and  the  edge  is  zigzag, 

up  and  down,  like  that  of  a  rhombohedron,  instead  of  hori- 

75.          zontal  as  in  the  pyramid.    Moreover  the  angles 

•i          between  the  faces  over  the  edges  which  meet 

^NL        in  the  vertex  are  only  alike  every  other  one — 

JB  Ik      in  other  words,  there  are  two  sets  of  three 

|A    each,  those  of  one  set  more  obtuse  than  those 

sjjm       of  the  other. 

'  a  The  two  hexagonal  prisms  before  described 

and  the  hexagonal  pyramid  of  the  diagonal 
series  also  belong  to  the  Rhombohedral  System. 

The  number  of  species  crystallizing  in  the  rhombohe- 
dral  division  of  the  Hexagonal  System  is  very  large,  and 
some  of  them,  as,  for  example,  calcite,  are  very  highly 
complex.  In  the  figures  of  calcite  given  here,  76  to  80, 


76. 


the  faces  r,f,  e  belong  to  different  rhombohedrons;  v  to  a 
scalenohedron ;  m  is  the  unit  hexagonal  prism ;  c  the  basal 
plane. 

Fig.  81  represents  a  more  complex  crystal,  also  of  cal- 
cite, and  Fig.  82  gives  a  basal  projection  of  it.  Here  there 
are  several  rhombohedrons,  r,  e,  0,/J  the  scalenohedrons 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    41 

v  and  t;  the  prism  m.  These  figures  show  well  the  sym- 
metry about  three  planes  meeting  in  angles  of  60°.  Fig. 
83  shows  a  crystal  of  hematite;  u  and  r  are  faces  of  two 


81. 


rhombohedrons,  and  n  faces  of  the  hexagonal  pyramid  of 
the  diagonal  series. 


IV.  Orthorhombic  System. 

The  characteristic  forms  of  the  Orthorhombic  System  are 
the  rhombic  prism  and  pyramid;  there  are  also  other  forms 
called  domes. 

Rhombic  Prism. — Fig.  84  shows  what  is  called  a  rhombic 
prism,  the  terminal  face  of  which  (formed  by  the  baaal 

84.  85. 


plane  c)  is  a  rhomb,  not  a  square  as  in  the  square  prism, 
which  it  somewhat  resembles.  The  angle  between  two 
faces  over  one  vertical  edge  is  obtuse,  or  greater  than  90°, 


42  MINERALS,  AND   HOW  TO  STUDY  THEH. 

the  other  acute  and  just  as  much  less  than  90°.  For  in- 
stance, if  the  angle  of  the  front  edge  is  100°,  the  angle 
of  the  side  edge  would  be  80°.  There  may  be  a  great 
many  rhombic  prisms  on  the  crystals  of  the  same  species, 
differing  in  the  angles  of  their  two  edges. 

Rhombic  Pyramid. — The  rhombic  pyramid  is  shown  in 
Fig.  85;  its  cross-section,  like  that  of  the  prism,  is  a  rhomb, 
and  its  edges  belong  in  three  sets,  with  different  angles 
for  each.  These  angles  are  characteristic  for  the  crystals 
of  a  given  species.  There  may  be  a  great  variety  of  rhom- 
bic pyramids,  differing  in  their  angles,  and  each  corre- 
sponding to  a  given  rhombic  prism. 

Fig.  86  shows  a  combination  form   belonging  to  this 
system  which  looks  like  a  cube  a  little,  and  resembles  it 
86.  in  that  the  angles  between  the  faces  are 

90°,  but  differs  because  the  faces  instead 
of  being  all  alike  belong  in  three  sets  of 
two  each.  It  is  for  this  reason  that  they 
are  lettered  a,  b,  c.  Of  these  c  is  called 
the  base,  and  a  and  b  are  called  pinacoids.  This  form  also 
resembles  the  second  or  diametral  prism  of  the  tetragonal 
system  (Figs.  45  and  50);  but  in  that  form  the  four  verti- 
cal faces  (a)  were  all  alike,  while  here,  as  has  been  stated, 
they  are  only  alike  two  and  two. 

Domes. — The  forms  shown  in  Figs.  87,  88  are  called 
domes,  from  the  Latin  for  house  (domus),  because  when 
they  meet  above  they  make  a  horizontal  edge  like  a  hip- 
roof. These  domes  are  often  and  very  conveniently  called 
horizontal  prisms;  that  of  Fig.  87  is  also  called  a  macro- 
dome,  because  the  faces  (I)  are  parallel  to  the  longer 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     43 


lateral  axis  (see  Fig.  84);  those  (e,  d)  of  Fig.  88  are 
similarly  called  brachydomes,  because  the  faces  are  paral- 
lel to  the  shorter  axis. 

Figs.  159-161,  of  sulphur,  on  p.  171,  show  some  simple 
orthorhombic  crystals;  the  faces  p  and  s  belong  to  rhom- 

87. 


bic  pyramids,  and  n  is  a  dome.  Fig.  89  shows  a  more 
complex  crystal,  and  Fig.  90  is  a  basal  section  of  the  same. 
Here  the  faces  lettered  e,  f  belong  to  two  rhombic  pyra- 
mids; d,  li,  k  to  domes;  a,  b  are  pinacoids,  and  c  is  the 


All  these  rhombic  forms  belong  in  the  ORTHORHOMBIC 

90. 


SYSTEM,  'n  which  the  planes  are  referred  to  three  unequal 
axes  at  right  angles  to  each  other,  as  shown  in  Figs.  84, 
85,  86. 

It  will  be  seen  that  the  grouping  of  faces  about  the  ex- 
tremities of  each  of  the  axes  differs  from  that  about  the 


44 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


others,  but  there  are  three  planes  of  symmetry  parallel  to 
the  faces  a,  b,  c,  which,  however,  are  all  unlike. 


V.    Monoclinic  System. 

Oblique  Rhombic  Prism. — Fig.  91  shows  an  ohlique 
rhombic  prism  in  which  the  end  plane  is  rhombic  in  form, 
but  it  is  oblique  to  the  faces  of  the  prism,  instead  of  being 
at  right  angles  as  described  in  the  Orthorhombic  System. 
This  oblique  prism  and  a  variety  of  other  forms  of  the 
same  group  belong  to  the  MONOULINIC  SYSTEM,  in  which 
the  planes  are  referred  to  three  unequal  axes,  two  of  the 
angles  between  which  are  right  angles,  while  the  third  (in 
front)  is  obtuse.  In  Figs.  91  and  92  the  forms  resemble 


Figs.  84  and  86  somewhat,  but  the  top  or  basal  plane  c  is 
inclined  to  the  front  edge  of  m  (Fig.  91),  and  to  the  plane 
a  (Fig.  92),  not  at  right  angles  to  them;  the  faces  «,  b  are 
also  called  pinacoids,  while  m  is  the  prism. 

The  monoclinic  forms  are  too  difficult  to  be  described 
fully  here,  but  it  is  not  hard  to  learn  what  is  most  essential 
about  them.  Thus  in  Figs.  93  to  95  (of  pyroxene)  it  is 
seen  that  the  faces  are  in  pairs  alike  on  either  side  of  a 
middle  plane  parallel  to  the  face  lettered  b.  In  other 


THE  FOEMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.  45 


words,  this  middle  plane  parallel  to  b  is  a  single  plane  of 
symmetry,  the  only  one  existing.  The  forms  are  named 
much  as  in  the  Orthorhombic  System:  m  is  a  prism;  u,  v,o 


are  pyramids  (half -pyramids  since  each  face  occurs  only 
four  times);  a,  b  are  pinacoids,  and  c  is  the  basal  plane. 
Planes  on  the  alternately  obtuse 
and  acute  edges  between  cand  a 
(Fig.  92)  are  called  orthodomes; 
those  on  the  four  similar  edges 
between  c  and  b  are  clinodomes. 
Fig.  96  shows  a  basal  projec- 
tion of  a  more  complex  mono- 
clinic  crystal;  here  the  sym- 
metry parallel  to  the  faces  b  is 
clearly  exhibited.  Fig.  97,  of  datolite,  shows  another  com- 
plex monoclinic  crystal;  here  g,  mx)  are  clinodomes,  also 
u,  x,  orthodomes ;  n,  e,  X,  F,  q  are  all  pyramids. 

VI.     Triclinic  System. 

Finally  there  is  one  other  group  of  forms,  which  belong 
to  the  TRICLINIC  SYSTEM.  In  this  system  the  planes  are 
referred  to  three  unequal  axes  all  oblique  to  each  other. 
Here  all  the  intersections  are  oblique  and  the  like  faces 


46  MINERALS,  AND   HOW   TO    STUDY   THEM. 

are  in  pairs  only  on  opposite  sides  of  a  crystal;  there  is  no 
plane  of  symmetry  at  all. 

Fig.  98,  of  axinite,  and  Fig.  99,  of  albite  feldspar,  show  two 
triclinic  crystals.  Here  it  is  seen  that  the  like  planes  are 
in  sets  of  two  each,  one  in  front,  the  other  behind,  repre- 
sented in  dotted  lines.  In  Fig.  99  there  is  some  resem- 
blance to  a  monoclinic  crystal,  but  the  angle  between  the 
faces  b  and  c  is  not  90°  as  it  must  be  there;  and  moreover 
the  angles  bin,  bM  are  different,  as  are  also  the  angles  bo, 
bp.  Hence  m  and  M are  different  planes, and  also  o  and  j9, 


The  subject  of  triclinic  crystals  will  not  be  carried  further, 
because  of  its  great  difficulty. 

When  we  come  to  examine  a  large  number  of  crystals  of 
different  species,  we  are  at  first  discouraged  by  the  almost 
infinite  variety  which  we  find.  But  as  we  gain  in  experi- 
ence, much  of  this  difficulty  gradually  disappears  and  we 
become  able  to  classify  a  large  part  of  them  into  one  of  the 
six  groups  or  systems  which  have  been  mentioned.  To 
learn  all  about  crystals,  however,  is  a  difficult  matter,  and  it 
will  be  best  for  the  student  to  master  the  other  characters 
of  minerals,  and  to  become  well  acquainted  with  the  com- 
mon species,  before  taking  up  a  larger  book  to  study  the 
subject  of  crystallography  in  detail. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.  47 

MEASUREMENT  OF  ANGLES. 

It  is  a  help  in  studying  minerals  to  be  able  to  measure 
the  angles  between  the  faces  of  a  crystal,  for  in  this  way  it 
is  possible  to  tell  a  square  prism  from  a  rhombic  prism,  a 
cube  from  a  rhombohedron,  and  so  on.  The  simplest 
method,  when  the  crystal  is  large  enough,  is  to  use  the  in- 
strument shown  in  Fig.  100,  called  a  contact  goniometer. 

This  is  best  made  of  brass  with  a  circle  or  semi-circle, 
100. 


divided  accurately  into  degrees  and  half-degrees,  and  pro- 
vided with  two  arms  moving  on  a  pivot  at  the  center.  In 
Fig.  100  these  arms  are  fixed  at  the  center  o,  and  sup- 
ported by  om,  but  they  can  slide  forward  and  back  so  as  to 
accommodate  them  to  crystals  of  different  size.  The  crys- 
tal is  placed  between  the  jaws  a  and  c,  in  such  a  position 
that  the  two  faces  whose  angle  is  to  be  measured  are  ex- 
actly in  contact  with  them,  and  the  edge  between  these 
faces  is  at  right  angles  to  them.  The  angle  (measured 
from  the  zero,  0°,  at  Jc]  is  then  read  off  from  the  upper  edge 
of  d,  for  the  lines  from  d  and  k  pass  precisely  through  the 
center  o  and  are  parallel  to  the  edges  at  a  and  c. 


48  MINERALS,  AND   HOW   TO    STUDY   THEM. 

A  very  fair  substitute  for  an  expensive  goniometer  can 
be  made  from  a  cheap  protractor,  by  first  cutting  out  two 
101.  ^  arms  of  thin  wood  shaped  like 

the  steel  ones  of  Fig.  101,  and 
then  putting  through  a  brass 
pivot  on  which  they  can  turn.* 
It  is  not  necessary  nor  desir- 
able that  these  arms  should 
be  permanently  attached  to 
the  protractor.  One  pair  of  edges  (the  inner  edges  to  the 
right  in  the  figure)  must  be  exactly  in  line  with  the  center 
or  pivot;  between  them  the  angle  is  read  off  when  the 
arms  are  placed  on  the  protractor,  the  pin  then  passing 
through  its  center  and  one  edge  through  its  zero.  The 
other  pair  of  edges  (to  the  left)  must  be  parallel  to  those 
first  mentioned,  so  as  to  give  the  same  angle;  the  two  faces 
of  the  crystal  whose  angle  is  to  be  measured  are  placed  be- 
tween them,  as  before  explained. 

For  measuring  the  angles  between  the  faces  of  very 
small  crystals  an  instrument  called  a  reflecting  goniometer 
is  used.  This  is  described  in  larger  works.  It  is  some- 
what expensive,  and  its  use  requires  both  skill  and  ex- 
perience. It  demands,  moreover,  polished  faces  if  good 
results  are  to  be  obtained. 

IRREGULARITIES  OF  CRYSTALS. 

Distorted  Crystals. — Most  of  the  crystals  of  minerals 
would  give  a  very  poor  impression  of  nature's  workmanship 

*  One  of  the  small  brass  screws  used  to  fasten  together  sheets  of 
manuscript  can  be  employed. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    49 


to  one  who  expected  always  to  see  them  exactly  like 
carefully-made  models,  or  like  the  figures  given  on  the 
preceding  pages.  The  cubes  of  galena  which  we  find  are 
often  flattened  or  drawn  out.  An  octahedron  (Fig.  102) 
may  be  flattened  so  as  to  look  like  Fig.  103;  a  dodeca- 
hedron (Fig.  104)  may  take  the  forms  shown  in  Fig.  105  or 
106.  And  so  of  other  crystals.  But  it  is  not  really  to  oe 
supposed  that  the  forms  are  badly  made,  like  a  bad  model; 


102. 


103. 


on  the  contrary,  the  size  of  the  like  faces  on  a  crystal  may 

vary,  and  so  the  shape  of  the  solid  as  a  whole,  but  the  angles 

104.  105.  106. 


between  them  remain  the  same.  Moreover,  when  we  study 
a  crystal  more  carefully,  we  find  that  what  is  really  essential 
is  not  the  size  or  shape  of  each  face,  but  the  way  in  which 
the  little  molecules  of  which  the  whole  is  built  up  are 
arranged.  For  example,  in  a  cube  the  essential  point  is 
the  fact  that  the  structure  is  the  same  in  the  direction  of 
the  three  cubic  faces.  It  follows  from  this  that  in  the 
cube  not  only  are  the  angles  between  two  adjacent  faces 


50  MINERALS,  AND   HOW   TO   STUDY   THEM 

always  90°,  but  the  six  cubic  faces  are  all  similar;  and 
therefore  if  there  is  the  easy  fracture,  called  cleavage,  par- 
allel to  one  cubic  face,  there  will  be  the  same  cleavage  also 
parallel  to  the  others.  But  the  actual  size  of  the  faces  is 
a  matter  of  no  importance.  In  fact,  in  one  species  the 
cubes  are  sometimes  lengthened  so  as  to  be  like  fine  hairs. 
Similar  remarks  can  be  made  in  regard  to  the  distorted 
octahedron  and  dodecahedron  figured  above,  and  indeed 
about  any  distorted  form.  The  symmetry  in  the  molecu- 
lar structure,  and  hence  the  angles  between  the  faces,  re- 
main unchanged,  although  the  symmetry  of  the  external 
geometrical  form  is  not  that  of  the  ideal  crystal. 

Another  good  example  of  what  is  possible  in  a  distorted 
crystal  can  be  explained  by  referring  to  Fig.  107,  a  cube 
107.  with  octahedral  faces  on  its  solid  angles. 
Now  instead  of  this,  the  ideal  form,  it  is 
common  to  find  in  natural  crystals  no  two 
of  the  triangular  octahedral  faces  of  the 
same  size;  some  of  them  may  even  be  ab- 
sent; while  the  cubic  faces  vary  widely  also.  But  such  a 
crystal  is  not  essentially  different  from  Fig.  107,  for  every 
octahedral  face  is  identical  with  each  of  the  others  if  it  is 
equally  inclined  to  the  three  adjacent  cubic  faces,  that  is, 
to  the  three  crystallographic  axes,  even  if  the  faces  all 
differ  in  size.  In  other  words,  it  is  here,  as  always,  the 
position  of  the  face,  as  showing  the  kind  of  molecular 
structure,  not  its  size,  which  is  essential. 

In  the  same  way  a  cube  may  in  nature  look  like  a  square 
prism,  for  the  angles  between  the  faces  are  all  right  angles 
in  both  cases,  and  so  the  goniometer  will  not  tell  the  differ- 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     51 

ence  between  them,  as  has  been  already  explained;  but  the 
molecular  structure  of  the  two  is  not  to  be  confounded. 
In  the  square  prism  there  is  the  same  arrangement  in  the 
transverse  directions,  but  a  different  one  up  and  down; 
hence  the  square  top  of  the  crystal  is  not  like  the  four 
similar  oblong  vertical  faces,  and  we  may  find  cleavage 
parallel  to  one  set  and  not  to  the  other. 

From  the  fact  that  so  much  variation  is  possible  in  the 
size  of  the  like  faces  on  a  crystal,  and  hence  in  the  shape  of 
the  whole,  it  is  evident  that  the  practical  study  of  natural 
crystals  is  much  more  difficult  than  the  study  of  the  mod- 
els which  give  the  ideal  forms.  This  is  especially  true 
because  most  crystals  are  so  implanted  on  the  rock,  or  em- 
bedded in  it,  that  only  part  of  the  form  has  been  devel- 
oped. Thus  quartz  crystals  are  often  attached  at  one 
extremity,  while  only  the  other  end  Jias  had  a  chance  to 
grow  freely.  Or  the  crystals  may  be  implanted  upon  a 
surface  of  rock  so  that  only  a  series  of  minute  faces  and 
angles  are  visible.  In  such  cases  the  study  of  the  form  is 
really  a  difficult  matter  requiring  much  skill  and  experi- 
ence, and  the  beginner  should  not  be  discouraged  because 
he  cannot  tell  at  once  what  the  form  of  a  crystal  really  is. 
Even  here,  however,  some  conclusion  can  often  be  drawn 
from  the  shape  of  the  faces:  thus,  if  regular  triangles, 
they  probably  belong  to  an  octahedron;  or  if  rhombs,  to  a 
rhombic  dodecahedron;  and  so  on. 

Besides  the  crystals  that  have  been  just  spoken  of, 
which,  while  they  look  at  first  irregular,  are  really  perfect 
in  the  matter  of  the  position  of  the  faces  and  of  the  angles 
between  them,  there  are  others  which  are  really  deformed, 


MINERALS,  AND   HOW  TO   STUDY   THEM. 


109. 


Some  peculiar  conditions  attending  the  growth  of  the 
crystal,  or  perhaps  some  force  which  has  acted  upon  it 
since  it  was  formed,  has  resulted  in  bending  or  twisting  it 
108.  out  of  its  normal  shape,  so  that  it  may 

differ  widely  in  angle  from  the  regular 
form.  Thus  the  faces  may  be  curved,  as 
with  the  barrel-shaped  crystals  of  pyro- 
morphite,  or  like  the  peculiar  convex  faces 
common  on  crystals  of  the  diamond;  or  the  whole  crystal 
may  be  bent,  as  is  seen  sometimes  in  the  case  of  crystals  of 
quartz,  or  of  stibnite,  or  of  some  kinds  of  chlorite  (Fig. 
108). 

Or  again,  aside  from  this  curving  and  twisting,  a  crystal 
may  have  had  its  shape  more  or  less  changed  by  some  forco 
exerted  in  the  rock  since  it  was  made;  it 
may  even  have  been  broken  and  again 
cemented  together,  so  that  many  irregu- 
larities may  result.  Fig.  109  shows  a 
crystal  of  beryl  which  has  been  broken 
into  many  pieces,  these  slightly  displaced 
from  each  other,  and  the  whole  cemented 
together  by  quartz. 

Other  irregularities  of  crystals  besides 
that  mentioned  also  occur.  The  faces 
of  crystals,  instead  of  being  perfectly 
smooth,  are  often  rough,  perhaps  because 
made  up  of  a  multitude  of  crystal  points.  Or  they  may 
be  covered  with  fine  lines,  or  striations,  as  those  on  the 
cubic  faces  of  pyrite  (see  Fig.  183  on  p.  213),  which  are  ex- 
plained by  the  successive  combination  of  another  face  (the 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    53 

pyritohedron)  in  narrow  lines  with  the  cubic  face.  In 
Fig.  110  the  fine  lines  represent  striations 
on  a  dodecahedral  face  due  to  the  pres- 
ence also  of  the  octahedron.  This  oscil- 
latory combination,  as  it  is  called,  may 
go  so  far  as  to  make  the  crystal  nearly 
round,  like  some  prismatic  crystals  of 
tourmaline.  Striations  are  also  sometimes 
due  to  twinning,  as  is  common  with  the  triclinic  feldspars 

111.  (see  P-  59>  also  FiS-  2~3>  P-  289)-     -Fig-  m 

shows  an  octahedral  crystal  of  magnetite 
with  twinning  lamellae  appearing  as  stria- 
tions on  an  octahedral  face. 

Again,  the  faces  may  have  a  multitude 
of  little  elevations  or  depressions,  the 
latter  like  the  pits  spoken  of  on  p.  65  as 
produced  by  etching;  in  fact  their  presence  can  some- 
times be  explained  as  due  to  etching  by  nature.  The 
same  cause— the  action  of  some  partial  solvent  after  the 
formation  of  the  crystal— often  explains  the  rough  faces 
alluded  to.  Connected  with  the  subject  of  minute  eleva- 
tions on  the  face  of  a  crystal  is  the  replacing  of  the 
face  by  one  or  more  others  varying  a  very  little  from 
it  in  angular  position.  The  very  low  pyramids  seen 
on  some  cubes  of  English  fluorite  are  good  examples 
of  this  (see  Fig.  211,  p.  245).  Such  planes  are  often 
called  vicinal  planes  (from  the  Latin  vicinus,  neighbor- 
ing). 

Crystals  which  have  formed   rapidly  may  have  only  a 
more  or  less  regular  skeleton  shape,  like  the  crystal  of  salt 


54 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


represented  in  Fig.  112.     The  salt-crystals  sometimes  show 
112.  distinctly  one  face  only  with  the 

depression  in  the  center,  so  that 
they  are  called  hopper-shaped  crys- 
tals. The  cavernous  crystals  of 
pyromorphite  and  vanadinite  give 
other  examples.  Crystals,  often  for 
the  same  reason,  enclose  foreign 
substances,  sometimes  in  the  form  of  liquids,  as  the  quartz 
crystals  that  contain  water,  occasionally  with  a  movable 
bubble  of  air.  Or  the  liquid  may  be  carbon  dioxide,  then 
often  with  a  bubble  of  the  same  substance  in  the  form  of 
gas.  In  such  cases  the  crystal  must  have  been  formed 
under  great  pressure,  sufficient  to  keep  the  gas  in  the 
liquid  form.  Fragments  of  such  crystals  heated  in  the 
gas-flame  fly  to  pieces  with  great  violence,  because  of  the 
expansion  of  the  gas  formed  from  the  liquid  by  heat. 

Crystals,  too,  frequently  contain  foreign  solid  substances 
of  many  kinds;  quartz  crystals  thus  enclose  clay,  particles 
of  carbon,  etc.  The  famous  groups  of  large  calcite  crys- 
tals from  Fontainebleau  contain  some  sixty  per  cent  of 
sand;  it  is  most  remarkable  that  the  113. 

force  of  crystallization  was  powerful 
enough  to  marshal  into  place  the  cal- 
cite molecules  under  such  circum- 
stances. Sometimes  the  impurities' 
are  regularly  arranged  in  the  crystal, 
and  then  a  curious  effect  is  obtained 
in  a  cross-section  cut  and  polished. 
Fig.  113,  of  garnet  enclosing  quartz,  shows  this  well 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    55 

Another  interesting  example  is  afforded  by  the  variety  of 
andalusite  called  made  or  chiastolite.  Sections  obtained  at 
different  points  in  a  long  crystal,  when  polished,  may  show 
the  forms  given  in  Fig.  114. 

Pseudomorplis. — A  peculiar  class  of  false  forms,  in  which 
the  crystalline  shape  does  not  belong  to  the  chemical  sub- 


stance, must  be  briefly  described  here.  These  are  called 
pseudomorphs. 

Pseudomorph*  means  false  form,  and  the  name  is  ap- 
plied to  a  specimen  having  the  form  characteristic  of  one 
species  and  the  chemical  composition  of  another.  This 
seemingly  difficult  contradiction  is  easily  explained.  Most 
chemical  compounds  are  liable  to  undergo  a  change  or 
alteration  when  subjected  to  certain  conditions,  as  moist- 
ure, the  action  of  alkaline  waters  or  acid  vapors.  Thus 
oxide  of  copper,  the  mineral  cuprite  (CuaO),  is  lather 
easily  changed  chemically  to  the  carbonate  of  copper, 
called  malachite  (CuC03.Cu(OH)2);  the  calcium  sulphate, 
anhydrite  (CaSOJ,  assumes  water  and  changes  to  the 
hydrated  sulphate,  gypsum  (CaS04  +  2H20);  the  sulphide 
of  iron,  pyrite  (FeSa),  changes  to  the  hydrated  sesquioxide, 
limonite  (2Fe,Os.3H20). 

Now,  in  these  and  similar  cases,  if  the  original  mineral 
was  in  crystals  the  external  form  is  usually  preserved, 

*  From  rpfvdijS,  false,  and  juop(pt},form. 


56  MINERALS,  AND    HOW   TO   STUDY   THEM. 

often  perfectly,  while  the  chemical  nature  and  the  molec- 
ular structure  have  changed.  Hence  we  describe  the  false 
forms  mentioned  as  pseudomorphs  of  malachite  after  cu- 
prite, so  too  gypsum  after  anhydrite,  limonite  after  pyrite. 

Other  examples  are  pseudomorphs  of  chlorite  after 
garnet,  pyromorphite  after  galena,  kaolin  after  feldspar. 
In  a  few  rare  cases,  where  the  same  chemical  com- 
pound occurs  in  nature  in  two  distinct  crystalline  forms, 
each  with  its  own  molecular  structure,  a  change  may  take 
place  in  the  structure  of  one  of  these  minerals  without 
alteration  of  the  chemical  substance.  Thus  the  rare  min- 
eral brookite  (titanium  dioxide,  Ti02)  may  be  changed  to 
rutile  (also  TiOa).  Such  pseudomorphs  have  the  special 
name  of  paramorphs. 

The  cases  where  the  original  substance  has  entirely  dis- 
appeared and  some  other  has  come  in  to  take  its  place  are 
also  called  pseudomorphs.  Thus  we  occasionally  find  quartz 
in  the  form  of  calcite,  or  of  fluorite,  or  of  barite,  that  is, 
pseudomorph  after  one  of  these;  also  tin-stone  in  the  form 
of  orthoclase  feldspar;  native  copper  in  the  form  of  aragon- 
ite.  Even  fossil  wood  may  be  said  to  be  a  pseudomorph 
of  quartz  or  opal  after  the  original  wood,  the  structure  of 
which  it  sometimes  preserves  with  wonderful  perfection. 

GROUPINGS  OR  AGGREGATIONS  OF  CRYSTALS. 

Some  crystals  occur  isolated  and  alone,  and  then  the 
form  is  usually  developed  on  all  sides,  and  with  something 
of  the  regularity  which  the  ideal  model  shows.  Thus  we 
find  perfect  garnets  in  mica  schist  or  granite,  and  gypsum 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     57 

crystals  in  clay.  But  it  is  still  more  common  to  find 
crystals  grouped  together  either  irregularly,  as  in  the 
majority  of  cases,  or  perhaps  in  parallel  position,  or  again 
in  the  peculiar  way  called  twinning;  the  last  mentioned 
will  be  described  first. 

Twin  Crystals. — The  most  interesting  and  important 
case  of  the  grouping  of  crystals,  or  parts  of  crystals,  is 
shown  in  those  curious  complex  forms  called  TWINS. 
Fig.  115  represents  a  twinned  octahedron,  and  Fig.  116 
two  twinned  cubes.  In  the  first  case  the  growth  of  the 

116. 


crystal  as  a  whole  has  been  such  that  one  half  has  been 
developed  in  reversed  position  to  the  other,  as  if  it  had 
been  revolved  around  through  half  the  circumference  or 
180°,  and  this  about  an  axis  (called  the  twinning  axis)  at 
right  angles  to  two  opposite  octahedral  faces.*  This  is  de- 
scribed as  a  contact-twin. 

In  Fig.  116  the  two  cubes  interpenetrate  each  other,  but 
each  one  is  in  such  a  position  as  if  it  too  had  been  revolved 
180°  around  an  axis  running  through  two  diagonal  angles 
(the  same  octahedral  axis  as  in  the  first  case).  This  is 
called  a  penetration-twin. 

*  Such  a  line  joining  the  middle  points  of  two  opposite  octahedral 
faces  is  called  au  octahedral  axis. 


58 


MINERALS,    AND    HOW  TO   STUDY   THEM. 


These  two  cases  illustrate  all  that  is  most  essential 
about  a  twin.  In  every  case  the  two  crystals,  or  parts  of 
crystals,  are  in  such  a  position  that  one  seems  to  have  been 
turned  around  180°  with  reference  to  the  other,  and  this 
usually  about  an  axis  at  right  angles  to  some  simple  crys- 
talline plane,  which  is  then  called  the  t winning-plane. 

Fig.  1 17  shows  a  twin  crystal,  of  the  contact  type,  of 
cassiterite  or  tin-stone;  Fig.  118  one  of  columbite,  which 


118. 


122. 


also  illustrates  the  point  that  the  difference  in  direction  of 
the  striations  of  the  two  halves  shows  that  the  crystal  is 
twinned.  Fig.  119  again  is  a  penetration-twin  of  stauro- 
lite,  also  called  cross-stone.  Figs.  120,  121,  122  show 
what  are  called  repeated  twins.  These  last  are  often  very 
regular  in  form,  the  complex  or  twinned  crystal  being 
made  up  of  perhaps  three,  five,  six,  or  even  eight  parts  of 
crystals  or  complete  crystals,  symmetrically  arranged  so  as 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    59 

to  resemble  a  star  in  many  cases.     Fig.  123,  of  rutile,  shows 
another  kind  of  repeated  twinning. 

Still  again  the  twinning  may  be  repeated  in  the  form  of 
thin  parallel  layers,  each  in  reversed  position          133. 
to  the  next.     This  is  called  polysynthetic  twin- 
ning and  is  best  illustrated  by  a  piece  of  a 
triclinic  feldspar  showing  fine  lines  or  stria- 
tions  on  a  surface  of  .basal  cleavage;   these 
lines  are  simply  the  edges  of  the  thin  succes- 
sive parallel  plates.     By  holding  the  specimen  so  as  to 
catch  the  reflection  of  light  from  a  distant  window  and 
turning  it  through  a  very  small  angle  it  may  be  seen  that 
first  one  set  of  edges  reflect  together  and  then  the  other. 
This  is  illustrated  by  a  figure  of  albite  on  p.  289.   Fig.  Ill 
on  p.  53  shows  polysynthetic  twinning  lamellae  in  a  crys- 
tal of  magnetite. 

It  will  be  understood  that  the  revolution  spoken  of  has 
not  actually  taken  place  in  ordinary  cases;  the  rule  is 
simply  given  in  this  form  so  as  to  show  best  the  mathe- 
matical relations  in  position  of  the  two  parts.  Still  it  is 
most  interesting  to  note  that  in  a  few  cases  it  is  possible 
to  cause  the  molecules  of  part  of  a  crystal  to  change  their 
124  position  so  as  to  produce  twinning  arti- 

ficially, as  by  pressure  in  the  proper 
direction. 

Thus  Fig.  124  represents  a  cleavage 

^    piece  of  calcite  placed  with  its  obtuse 

edge  on  a  firm  surface  and  then  pressed 
by  a  knife  (not  too  sharp)  at  a.  Steady  uniform  pressure 
^rves  to  reverse  the  position  of  the  molecules  in  the  part 


\ 


80 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


lying  to  the  right  so  that  the  whole  is  pushed  to  the 
side  and  assumes  a  twinned  position  with  reference  to  the 
rest.  If  skillfully  done,  no  change  in  the  transparency 
of  this  part  takes  place  and  the  new  surface  gee  is  per- 
fectly smooth.  In  nature  pressure  may  have  produced 
twinning  after  the  formation  of  the  crystal;  it  is  then 
called  secondary  twinning.  The  twinning  layers  or  la- 
mellae observed  in  most  cleavage  masses  of  otherwise  clear 
calcite  are  often  to  be  explained  in  this  way.  This  may 
also  be  true  in  some  cases  of  the  similar  layers  common  on 
large  crystals  of  pyroxene  and  which  cause  a  separation  or 
"  parting"  parallel  to  the  basal  plane,  appearing  much  like 
the  easy  fracture  called  cleavage. 

It  is  evident  that,  since  the  crystals  on  a  single  specimen 
of  a  species  may  be  grouped  in  a  great  variety  of  ways, 
it  is  not  always  easy  to  decide  whether  a  given  case  is  a 
twin  or  not;  this  often  becomes  a  matter  requiring  careful 
study,  exact  measurement  of  angles,  and  calculation,  per- 
haps also  of  optical  study.  For  example,  it  is  common  to 
find  quartz  crystals  crossing  each  other 
at  a  great  variety  of  angles,  but  real  twins 
like  Fig.  125  are  rare.  The  two  parts  of 
a  true  twin  are  usually  symmetrical  with 
reference  to  the  twinning  plane,  and 
there  is  always  the  reversal  of  one  half 
in  the  way  described. 

Parallel  Grouping. — One  very  com- 
mon case  of  the  grouping  of  crystals,  which  the  beginner 
is  apt  to  confound  with  twinning,  is  where  the  crystals  or 
parts  of  crystals  are  parallel  to  each  other,  so  that  the  axes 


125. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    61 

of  all  have  the  same  directions,  and  are  not  inclined  as  in 
most  twins.  This  is  illustrated  by  a  pile  of  cubes  with  faces 
parallel  and  having  re-entrant  angles  between  them.  The 
crystals  of  many  species  are  at  times  arranged  in  this  way, 
but  in  every  case  it  will  be  found  that  if  the  complex  group 
is  held  so  as  to  reflect  the  light  from  a  window,  the  faces  in 
adjoining  crystals  which  reflect  at  the  same  time  are  always 
similar  faces.  An  octahedron  of  fluorite,  built  up  of  a 
multitude  of  little  cubes  in  parallel  position,  is  a  not  very 
rare  example  of  this.  Fig.  126  shows  a  complex  crystal  of 
analcite,  formed  of  a  number  of  single  crystals  all  parallel; 
126.  127.  128. 


it  is  hence  not  a  twin.  The  large  and  complex  crystal  of 
quartz  which  forms  the  frontispiece  illustrates  well  this 
parallel  grouping  in  the  subordinate  parts. 

Parallel  grouping  is  most  interesting  when  the  result 
is  to  build  up  a  compound  form  with  branching  and 
rebranching  parts  like  the  limbs  of  a  shrub  or  tree,  and 
hence  giving  rise  to  a  kind  of  structure  called  arborescent 
or  dendritic;  here  all  the  crystals  or  parts  of  129. 

crystals  have  their  axes  in  the  same  direction. 
This  is  shown  in  Fig.  127,  of  native  copper. 
Fig.  128  is  similar,  though  here  there  is  also 
some  twinning,  but  not  distinctly  shown  on  so  small  a  scale. 


62  MINERALS,  AND   HOW  TO  STUDY  THEM. 

In  Fig.  129  the  little  plates  of  hematite  are  grouped 
together  so  as  to  form  a  large  crystal,  but  with  such  varia- 
tion in  their  position  that  the  top  has  the  shape  of  a  rosette. 
Such  a  crystal  is  called  by  the  Germans  an  Eisenrose,  or 
iron-rose. 

Another  curious  and  interesting  case  is  where  a  number 
of  crystals  are  implanted  upon  the  surface  of  another 
which  has  obviously  so  influenced  their  growth  that  they 
are  in  parallel  groups,  and  in  a  definite  position  relative 
to  it.  Many  cases  of  this  have  been  noted,  as,  for  example, 
the  rutile  crystals  on  a  tabular  crystal  of  hematite,  as  shown 
in  Fig.  130.  Fig.  131  is  a  related  case;  it  consists  now  of 
rutile  alone,  and  has  been  described  as  a  pseudomorph  (see 
p.  55)  of  rutile  after  hematite.  In  the  natural  healing  of 

130.  131. 


the  broken  surface  of  a  crystal,  as  of  quartz  (alluded  to  on 
p.  20),  it  follows,  almost  as  a  matter  of  course,  that  the  new 
molecular  growths  should  be  in  directions  parallel  with 
the  old  ones. 

The  common  method  of  grouping  of  crystals,  however, 
is  quite  irregular,  and  it  is  only  exceptionally  that  twins 
or  parallel  groupings  are  noted.  And  here  a  few  terms 
often  used  in  describing  specimens  must  be  explained; 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     63 

they  are  given  to  describe  the  way  the  crystals  or  crystal- 
line parts  are  aggregated. 

If  the  crystals  are  very  small  and  closely  clustered,  giv- 
ing a  rough  surface  a  little  like  coarse  sandpaper,  it  is 
said  to  be  drusy.  Or  the  crystals  may  cross  each  other 
more  or  less  regularly  like  the  meshes  of  a  net,  and  this 
form  is  said  to  be  reticulated.  When  the  crystals  or  crys- 
talline fibers  making  up  a  mass  are  very  slender,  some- 
thing like  needles,  the  structure  is  said  to  be  acicular;  if 
they  are  fine  like  hairs,  and  perhaps  found  in  a  tangled 
wad,  it  is  said  to  be  capillary;  if  fine  and  soft  like  moss, 
mossy. 

STRUCTURE  IN  GENERAL. 

Minerals  are  not  always  in  distinct  crystals,  like  those  of 
garnet  or  quartz,  or  even  in  aggregates  of  crystals.  On  the 
contrary,  many  of  the  specimens  in  a  mineral  cabinet  show 
no  crystalline  faces  at  all,  and  then  they  are  simply  called 
masxive.  There  are,  however,  important  distinctions  of 
structure  between  massive  minerals. 

First  of  all,  the  distinction  between  crystalline  and  amor- 
phous must  be  well  understood.  A  piece  of  clear  quartz, 
or  rock  crystal  as  it  is  often  called,  is  said  to  be  crystal- 
line; a  piece  of  glass  which  very  likely  the  eye  alone  could 
not  distinguish  from  it  is  amorphous  or  formless.  For 
the  mass  of  quartz,  though  it  has  no  definite  external  form, 
but  is  bounded  only  by  irregular  fracture  surfaces,  is  just 
as  truly  crystalline  in  structure  as  a  perfect  crystal;  in 
fact  it  may  be  itself  a  fragment  of  a  large  crystal.  This  is 
true  because  the  essential  idea  about  a  crystallized  mineral 


64 


MINERALS,  AND   HOW  TO   STUDY   THEM. 


132. 


is  that  there  should  be  the  regular  arrangement  of  the 
molecules  out  of  which  it  is  built  up.  It  is  not  always 
easy,  often  impossible,  with  the  eye  alone  to  decide  whether 
this  regularity  of  structure  exists.  It  is  shown  by  the 
cleavage,  as  will  be  explained  in  a  later  part  of  this  chap- 
ter; but  when  there  is  no  cleavage,  it  is  usually  by  optical 
examination  in  what  is  called  polarized  light  that  this 
can  be  most  easily  proved.  For  example,  the  bright  colors 
given  by  a  thin  fragment  of  a  quartz  crystal  in  polarized 
light  shows  at  once,  to  one  who  understands  the  subject 
of  optics,  that  it  is  crystalline  in  structure. 

In  the  glass,  on  the  other  hand,  the  molecules  have  no 
definite  arrangement  at  all,  and  hence  no  action  on  po- 
larized light.  It  is  somewhat  like  the 
difference  between  a  pyramid  built  of 
blocks  of  stone  laid  in  regular  rows  or 
after  a  more  intricate  pattern,  and  one  in 
which  the  blocks  are  tumbled  in  without 
order — only  here  we  should  need  mortar 
to  cement  the  whole  together,  while  na- 
ture's molecules,  in  either  case,  are 
bound  together  by  the  force  of  cohesion. 
It  is  interesting  to  note  here  one  of  the  ways  which  the 
skillful  mineralogist  has  of  finding  out  how  the  molecules 
of  a  crystal  or  crystalline  mass  are  arranged :  and  this 
may  be  a  very  important  matter,  for  next  to  the  chemical 
substance  this  molecular  structure  is  the  most  essential 
character  of  a  mineral.  He  does  this  by  what  he  calls  etch- 
ing, that  is,  by  allowing  some  liquid  (or  sometimes  a  gas), 
which  has  the  power  of  dissolving  the  substance  examined, 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     65 

co  act  upon  a  smooth  surface  for  a  short  time.  Then 
it  is  removed,  the  surface  washed  off  and  examined  under 
the  microscope.  Often  a  multitude  of  little  cavities  or  pits 
are  found  on  the  surface  whose  shape  shows  clearly  how 
the  molecules  are  built  up.  It  is  very  much  as  if  the 
stones  of  the  pyramid  spoken  of  were  so  smooth  and 
closely  fitted  that  no  joints  were  visible,  and  the  mason 
sbould  go  to  work  and  pull  out  a  number  till  he  could  see 
what  the  pattern  was.  Fig.  132  shows  the  etching-figures 
on  the  faces  of  a  crystal  of  quartz;  the  variation  in  their 
form  reveals  the  complex  structure  which  is  described 
further  on  p.  275. 

To  continue  the  discussion  of  the  structure  of  massive 
minerals. — In  general  it  can  be  said  that  such  specimens 
are  crystalline  in  structure  even  when  there  is  no  distinct 
evidence  of  it  to  the  eye,  for  the  amorphous  glasslike 
condition  is  the  exception,  not  the  rule.  There  are, 
however,  cases  in  which  the  structure  seems  to  be  inter- 
mediate between  the  distinct  crystalline  and  the  amor- 
phous, and  this  is  called  crypto-crystalline,  as  is  true  of 
many  of  the  massive  kinds  of  quartz.  Among  the  speci- 
mens of  massive  crystalline  minerals  there  are  a  great 
variety  of  different  kinds  of  structure,  and  it  is  necessary 
to  become  acquainted  with  the  terms  used  in  describing 
these. 

It  has  been  stated  that  the  easy  fracture  in  certain 
directions,  called  cleavage,  is  the  surest  and  easiest  way 
of  showing  that  a  massive  mineral  is  crystalline  in  struc- 
ture. Such  a  mass  is  said  to  be  cleavable,  which  means 
capable  of  being  cleaved,  or  showing  cleavage.  The  kinds 


66  MINERALS,  AND   HOW  TO  STUDY  THFM. 

of  cleavage  are  spoken  of  more  fully  on  pp.  70-72.  This 
cleavage  proves  that  the  mass  is  crystallized,  and  also 
often  reveals  what  sort  of  structure  it  has.  The  cleavage 
may  be  in  the  same  direction  in  all  parts  of  the  mass,  as 
if  it  were  a  piece  of  one  large  crystal;  or,  more  commonly, 
the  cleavage  directions  may  be  the  same  in  one  little  spot 
only— that  is,  for  each  individual  grain.  In  the  second 
case  the  mass  is  obviously  crystalline  and  the  structure  is 
said  to  be  cleavable  and  granular.  Such  a  mass  of  galena 
is  really  made  up  of  a  multitude  of  little  grains,  each  one 
of  which  has  its  own  directions  of  cleavage  and  presents 
to  the  eye  its  own  edges.  If  the  individual  grains  are 
large,  the  structure  is  said  to  be  coarse-granular}  and  if 
small,  it  is  fine-granular.  From  the  latter  we  pass  to 
the  closely  compact  kinds  in  which  the  structure  is  some- 
times not  obvious  to  the  eye  at  all;  they  may  then  be  said 
to  be  impalpable.  But  this  extreme  is  rare;  for  example, 
a  piece  of  white  marble,  even  if  it  is  so  fine-grained 
that  the  particles  cannot  be  seen  by  the  eye,  usually 
sparkles  in  a  strong  light  from  the  reflection  of  the  mul- 
titude of  minute  cleavage-faces. 

This  granular  structure  may  belong  also  to  other  min- 
erals which  do  not  have  the  distinct  evidence  of  crystalline 
structure  in  the  cleavage,  and  then  the  grains  may  often 
be  more  or  less  easily  separated  from  one  another;  some- 
times, as  in  granular  kinds  of  pyroxene,  they  are  found  to 
be  imperfect  crystals. 

Again,  if  the  mass  seems  to  be  made  up  of  layers, 
whether  separable  or  not,  it  is  called  lamellar.  If  it  is 
in  thin  leaves  or  plates,  which  can  be  separated  from  one 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.     67 

another,  it  is  called  foliated,  as  with  graphite.  It  is 
further  called  micaceous  when  the  separation  takes  place 
as  readily  as  in  a  piece  of  mica. 

If  the  mass  is  made  up  of  little  columns,  it  is  called 
columnar.  When  there  are  distinct  fibers,  it  is  said  to  be 
fibrous;  asbestus  is  said  to  be  fine-fibrous,  and  with  it  the 
fibers  are  easy  to  pull  apart,  or  separable,  but  this  is  not 
always  the  case.  There  are  many  intermediate  kinds 
between  the  fine-fibrous  and  coarse-columnar. 

If  the  fibers,  or  little  columns,  or  leaves  all  go  out  from 
a  center  like  the  spokes  of  a  wheel,  the  structure  is  said  to 
be  radiated,  as  wavellite  (Fig.  133),  or  perhaps  stellate 
when  it  is  star-shaped,  as  gypsum  (Fig.  134).  If  the 
layers  are  arranged  in  parallel  position  about  one  or  more 
centers,  the  structure  is  said  to  be  concentric;  as  malachite 
(Fig.  135).  All  these  terms,  granular,  foliated,  lamellar, 
columnar,  fibrous,  radiated,  etc.,  ordinarily  refer  to  the 
structure  of  the  mass  when  it  is  more  or  less  distinctly 
crystalline. 

When  the  external  surface  of  the  mineral  is  in  the 
form  of  rather  large  rounded  prominences,  it  is  called 
mammillary,  as  malachite  (Fig.  136);  if  the  prominences 
are  smaller,  somewhat  resembling  a  bunch  of  grapes,  it  is 
called  botryoidal,  as  prehnite,  smithsonite,  and  chalcedony 
(Fig.  137).  If  the  surface  is  made  up  of  little  spheres  or 
globules,  it  is  called  globular,  as  hyalite  or  prehnite  (Fig. 
138).  If  the  form  is  that  of  a  kidney,  it  is  called  reni- 
form,  as  hematite  (Fig.  139).  It  will  be  understood  that 
there  is  no  sharp  line  dividing  these  different  cases. 

Sometimes  the    mineral   takes   the  form  of  a  delicate 


68  MINERALS,  AND  HOW  TO  STUDY  THEM. 

133.  134. 


tes.  133  to  140  varieties  of  structure:  133,  Wavellite,  radiated;  134,  Gypsum, 
stellate  ;  135,  Malachite,  concentric;  130,  Malachite,  mammilla,,/;  137  Chall 


,  , 

cedony.  botryoidal  ;  138,  Prehnite,  globular  ;  139,  Hematite,  reniform  •  140 
,  stalactitic. 


THE  FORMS  OF  CRYSTALS  AND  KINDS  OF  STRUCTURE.    69 


branching  coral,  and  it  is  then  called  coralloidal,  as  cer- 
tain varieties  of  aragonite  (which  see).     If  made  up  of 


141. 


Concretions  from  Clay-beds. 

forms  like  small  stalactites,  it  is  said  to  be  stalactitic,  as 
limonite  (Fig.  140). 

If  the  material  has  clustered  about  a  center  like  those 
curious  forms  of  impure  calcium  carbonate — called  con- 
cretions —  common  in  the 
clay  (Fig.  141),  it  is  named 
concretionary.  Dendrites,  or 
dendritic  forms,  are  those 
which  have  more  or  less  the 
shape  of  a  branching  shrub  or 
tree,  as  the  forms  of  manga- 
nese oxide  (Fig.  142)  often 
seen  on  surfaces  of  smooth 
limestone  or  inclosed  in  moss- 
agates.  Some  dendritic  forms 
are  made  up  of  little  crystals  grouped  together  in  parallel 
position  as  remarked  on  p.  61. 


70  MINERALS,  AND  HOW  TO  STUDY  THEM. 


CHAPTER  IV. 
THE  OTHER  PHYSICAL  CHARACTERS  OF  MINERALS. 

BESIDES  the  external  form  of  minerals  shown  in  th« 
crystals  there  are  also  a  series  of  other  physical  characters, 
based  (1)  upon  the  molecular  force  of  cohesion,  (2)  upon 
density,  and  (3)  upon  the  action  of  light;  also  other 
characters  of  less  frequent  use,  depending  upon  heat,  elec- 
tricity, magnetism ;  finally,  a  few  minerals  have  distinctive 
characters  of  taste  and  odor. 

1.    CHARACTERS  DEPENDING  UPON  COHESION. 

The  characters  depending  upon  the  molecular  force  of 
cohesion  will  be  described  first.  These  include  the  cleav- 
age, fracture,  hardness,  tenacity,  and  elasticity. 

Cleavage. 

The  forms  of  crystals,  as  has  been  repeatedly  stated,  de- 
pend upon  their  molecular  structure;  but  this  internal 
structure  reveals  itself  also  by  the  cleavage,  or  the  natural 
easy  fracture  which  yields  more  or  less  smooth  faces  in  cer- 
tain definite  directions.  A  cleavage  surface  marks  a  direc- 
tion in  which  the  force  binding  the  molecules  together  is 
relatively  weak.  Thus  galena  is  said  to  have  cubic  cleavage 
because  its  molecules  separate  readily  in  a  direction  parallel 


THE   OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     71 

to  each  pair  of  parallel  cubic  faces;  in  other  words,  when 
fractured  it  breaks  into  a  multitude  of  little  cubes  (Fig. 
143).  Fluorite,  or  fluor-spar,  has  octahedral  cleavage,  since 
the  separation  of  the  molecules  is  143. 

easy  parallel  to  each  pair  of  octa- 
hedral faces;  hence  from  a  single 
mass  we  may  with  care  break  out 
an  octahedron.  This  cleavage  oc- 
tahedron, by  the  way,  is  readily 
distinguished  by  careful  examina- 
tion from  a  real  octahedral  crys-  Cubic  cieavage-Gaiena. 
tal,  because  the  faces  are  uneven  and  splintery,  not  uni- 
form like  the  normal  faces  of  a  crystal.  Even  when  the 
faces  of  a  crystal  are  rough  and  uneven,  they  are  quite  dif- 
ferent from  the  surfaces  formed  by  cleavage,  however  per- 
fect. 

The  octahedral  cleavage  of  fluorite  is  also  seen  in  the 
case  of  a  cubic  crystal,  since  its  solid  angles  may  be  easily 
broken  off,  giving  a  form  like  Fig.  107  on  p.  50,  which  we 
have  learned  is  a  cubic  modified,  or  with  its  angles  replaced, 
by  the  planes  of  an  octahedron.  Again,  from  a  piece  of 
sphalerite  or  zinc  blende  a  dodecahedron  may  sometimes 
be  broken  out  because  of  its  perfect  dodecahedral  cleavage; 
or  if  this  is  difficult  because  the  fragment  having  like 
cleavage  directions  is  too  small,  the  skillful  observer  can 
yet  prove  that  the  cleavage-faces  have  the  position  of  dode- 
cahedral planes  and  make  angles  of  60°  and  120°  with  each 
other.  Further,  a  piece  of  calcite  breaks  readily  into  a 
number  of  rhombohedrons,  all  of  the  same  angle,  and  it  is 
hence  said  to  have  perfect  rhombohedral  cleavage  (Fig.  144). 


72  MINERALS,  AND   HOW   TO    STUDY   THEM. 

In  the  same  way  we  find  that  amphibole  has  prismatic 
cleavage;  mica  has  highly  perfect  basal  cleavage  or  cleav- 
age parallel  to  the  end  or  basal  plane,  yielding  excessively 
144.  thin  sheets;  topaz  has  also 

basal  cleavage.  Gypsum  has 
perfect  cleavage  parallel  to 
the  side  plane  of  the  crys- 
tal, yielding  plates  almost  as 
thin  as  those  of  mica;  these 
plates  show  two  other  cleav- 
i  Cieavage-Caiche.  ages  on  the  edges,  but  dif- 
ferent in  character  from  each  other,  as  is  more  fully 
described  under  the  description  of  this  species.  Feldspar 
shows  two  cleavages,  both  nearly  perfect  but  one  a  little 
more  so  than  the  other,  and  these  make  an  angle  of  90°  or 
nearly  90°  with  each  other.  All  these  are  cases  of  perfect 
cleavage,  and  it  will  be  at  once  seen  how  important  a 
character  the  cleavage  is. 

But  the  cleavage  is  not  always  perfect,  as  in  the  exam- 
ples given;  sometimes  it  is  obtained  with  difficulty,  or  the 
surfaces  yielded  may  be  only  partially  smooth;  in  such 
cases  it  is  said  to  be  imperfect,  or  interrupted,  or  difficult. 
Occasionally  cleavage  exists  in  cases  where  it  is  so  hard  to 
obtain  that  it  is  ordinarily  not  noted  at  all.  Thus  crystal- 
lized quartz  usually  shows  only  a  conchoidal  fracture,  and 
the  absence  of  cleavage  is  a  character  which  at  once  dis- 
tinguishes it  from  the  feldspar  with  which  it  is  often 
associated ;  yet  a  crystal  of  quartz  which  after  being  heated 
has  been  plunged  into  cold  water  often  shows  cleavage 
parallel  to  the  pyramidal  planes,  perhaps  also  parallel  to 


THE   OTHER   PHYSICAL  CHARACTERS   OF   MINERALS.     73 

the   prism.      Cleavage-rifts  in  these  directions  are  some- 
times seen  in  natural  crystals. 

It  is  a  help  to  remember  that,  even  if  a  crystal  does  not 
actually  show  broken  surfaces,  the  cleavage  is  often  clearly 
indicated  by  a  fine  pearly  luster  on  the  face  of  the  crystal 
parallel  to  it.  This  is  seen  on  the  basal  plane  of  apophyl- 
lite,  on  the  side  plane  of  stilbite,  and  in  many  other  cases. 
This  pearly  luster  is  due  to  the  presence  of  cleavage  rifts, 
though  the  crystal  has  not  actually  parted,  just  as  a  pile  of 
thin  glass  sheets  shows  a  pearly  luster  on  top  because  of 
the  repeated  reflections.  Similarly 
the  cleavage  rifts  can  often  be  seen 
in  a  transparent  crystal;  a  flat  clear 
crystal  of  barite  or  of  celestite  (Fig. 
145)  often  shows  the  prismatic  cleav- 
age in  two  directions  making  an 
angle  of  about  104°  with  each  other. 

A  massive  specimen  of  a  mineral 
may  show  cleavage  as  a  multitude  of 
little  smooth  faces  changing  position 
with  that  of  the  grains  to  which  they 
belong;  if  these  grains  are  very  small, 
the  cleavage  may  appear  only  as  a  fine  spangling  of  the 
surface,  as  was  mentioned  on  p.  66. 

Fracture. 

The  nature  of  the  surface  given  by  fracture,  when  not 
the  smooth  surface  of  cleavage,  is  often  an  important  char- 
acter to  note,  especially  in  distinguishing  the  varieties  of  a 


74  MINERALS,  AND  HOW  TO  STUDY  THEM. 

mineral  species.  Thus  glass,  as  well  as  quartz  and  many 
mineral  species,  shows  a  shell-like  fracture  surface  which 
is  called  conchoidal  (Fig.  146),  or,  if  less  distinct,  small 
conchoidal,  or  subconchoidal. 


146. 


Conchoidal  Fracture— Obsidian  or  Volcanic  Glass. 


More  commonly  the  fracture  is  simply  said  to  be  uneven, 
the  surface  is  rough  and  irregular.  Occasionally  it  is 
hackly,  like  a  piece  of  fractured  iron.  Earthy  and  splin- 
tery are  other  terms  sometimes  used  and  easily  under- 
stood. 

Hardness  and  Tenacity. 

By  hardness  the  mineralogist  understands  the  degree  of 
resistance  which  the  smooth  surface  of  a  mineral  offers  to  a 
point  or  edge  tending  to  scratch  it.  A  diamond  easily 
makes  a  scratch  on  a  smooth  topaz  crystal;  the  topaz 
scratches  a  quartz  crystal,  while  the  quartz  scratches  a 
glass  surface,  and  the  glass  in  turn  scratches  one  of  cal- 
cite.  This  means  that  each  substance  named  is  harder 


THE   OTHER   PHYSICAL   CHARACTERS    OF  MINERALS.     75 

than  that  which  it  scratches,  or,  in   other  words,  softer 
than  the  one  by  which  it  is  scratched.* 

Mineralogists  have  found  it  convenient  to  select  a  num- 
ber of  minerals  for  the  comparison  of  hardness,  and  they 
are  designated  by  the  numbers  from  1  to  10,  as  given  in  the 
following  list.  Crystallized  varieties  are  to  be  taken  in 
each  case,  that  is,  a  crystal  with  even  surfaces  or  a  smooth 
cleavage  fragment. 

1.  Talc.  6.  Orthoclase. 

2.  Gypsum.  7.  Quartz. 

3.  Calcite.  8.  Topaz. 

4.  Fluorite.  9.  Corundum. 

5.  Apatite.  10.  Diamond. 

When  it  is  said  that  the  hardness  of  a  mineral  is  4,  this 
means  that  it  is  scratched  as  easily  as  fluorite,  for  example 
by  a  mineral  which  follows  in  the  list,  while  it  will  itself 
scratch  all  of  the  minerals  which  precede.  If  the  hard- 
ness of  a  mineral  is  given  as  5.5,  this  means  that  it  is  a 
little  harder  than  apatite  and  a  little  less  hard  than  ortho- 
clase. 

The  student  should  practise  with  the  minerals  in  this 
scale  up  to  topaz  or  corundum,  and  then  with  them  experi- 
ment upon  some  other  known  minerals,  until  he  learns  just 
what  degree  of  hardness  each  of  the  numbers  corresponds 
to,  especially  those  up  to  7.  He  ought  soon  to  become  so 
proficient  as  to  be  able  to  determine  the  lower  grades  of 

*  In  general  a  faint  scratch  can  be  made  on  the  surface  of  a  crys- 
tal by  the  edge  of  another  of  the  same  species;  this  is  readily  proved 
with  quartz. 


76  MINERALS,  AND   HOW  TO  STUDY  THEM. 

hardness  by  his  knife  without  the  use  of  the  reference 
species  at  all. 

He  will  find  at  once  the  following  general  distinctiona 
between  minerals  of  the  several  grades: 

No.  1 :  has  a  soft,  greasy  feel  in  the  hand,  like  talc  and 
graphite. 

No.  2:  can  be  scratched  easily  by  the  finger-nail,  as  a 
cleavage-piece  of  gypsum. 

No.  3 :  can  be  easily  cut  by  the  knife,  but  is  not  scratched 
by  the  nail. 

No.  4:  is  scratched  by  the  knife  without  difficulty,  but 
not  easily  cut  like  calcite. 

No.  5 :  is  a  little  hard  to  scratch  with  a  knife. 

No.  6:  is  hardly  touched  if  at  all  by  the  knife,  but  it  will 
scratch  ordinary  glass. 

No.  7:  scratches  glass  easily,  but  is  scratched  by  topaz 
and  a  few  other  minerals  mentioned  in  the  list  given  in 
Chapter  VIII.  The  minerals  which  are  as  hard  as  or  harder 
than  quartz  are  few  and  include  all  the  highly-prized  gems. 
Some  further  notes  on  hardness  are  given  in  connection 
with  the  list  referred  to. 

The  beginner  will  need  a  word  of  advice  in  regard  to 
testing  for  hardness.  In  the  first  place  treat  a  mineral, 
especially  a  crystal,  with  as  much  consideration  as  possible. 
A  scratch  on  a  piece  of  plate-glass,  like  a  daub  with  a  paint- 
brush on  a  white  wall,  is  a  little  thing,  but  it  may  have 
a  sadly  disfiguring  effect  and  make  a  pane  worth  when 
new  a  thousand  dollars  unsalable  except  to  be  cut  up. 
So  a  scratch  on  a  crystal  disfigures  it  and  destroys  its  value 
in  large  measure  in  the  eyes  of  one  who  is  a  true  mineral- 


THE    OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     77 

ogist.  Hence  make  as  minute  a  scratch  as  possible,  not 
longer  than  this  — ,  and  if  possible  put  the  scratch,  not  on 
the  most  prominent  surface,  but  where  it  will  show  least. 
Treat  the  crystal,  then,  as  if  it  had  feelings  to  be  hurt  by 
the  cut,  and  never  scratch  its  smooth  surface  wantonly, 
nor  if  it  is  possible  to  obtain  the  desired  information  with- 
out it. 

On  the  other  hand,  it  is  necessary  to  be  sure  and  distin- 
guish between  a  real  scratch  on  a  smooth  surface  and  the 
crushing  of  a  rough  surface  by  the  knife-edge;  a  very 
hard  mineral  may  often  be  scratched  in  this  way.  The 
danger  of  making  a  mistake  of  this  kind  is  made  less  if, 
besides  the  useful  knife-point,  the  mineral  be  rubbed  on  a 
piece  of  glass;  better  have  a  piece  at  hand  (not  disfigure  a 
window-pane).  Only  do  not  make  the  opposite  mistake  and 
call  a  white  ridge  left  by  a  soft  mineral  on  the  glass,  which 
can  be  easily  rubbed  off,  a  scratch. 

Once  more,  it  is  necessary  to  remember  that  minerals 
are  often  altered,  as  the  chemist  says;  that  is,  they  may 
have  undergone  some  chemical  change,  particularly  on  the 
surface,  which  has  rendered  that  soft  when  the  original 
mineral  was  really  hard.  Thus  it  is  often  easy  to  make  a 
scratch  on  a  crystal  of  corundum  because  of  a  little  sur- 
face change,  while  the  mass  within  is  very  hard.  If  the 
mineral  is  used  to  scratch  with,  the  danger  of  a  mistake 
here  is  lessened. 

There  are  also  some  other  characters  depending  upon 
the  force  of  cohesion  acting  between  the  molecules  of  a 
mineral.  These  include  the  following,  which  are  some- 
times grouped  under  the  general  head  of  TENACITY: 


78  MINERALS,  AND   HOW  TO   STUDY   THEM. 

Malleable:  capable  of  being  flattened  out  under  the 
blow  of  a  hammer  without  breaking  or  crumbling  into 
fragments.  This  is  conspicuously  true  of  gold  and  silver, 
and  makes  it  possible  to  beat  out  gold  into  leaves  of 
extreme  thinness.  The  property  of  malleability  belongs 
only  to  the  native  metals  and,  in  an  inferior  degree,  to  a 
few  compounds  of  silver. 

Ductile:  capable  of  being  changed  in  shape  by  press- 
ure, especially  of  being  drawn  out  into  the  form  of  wire. 
This  is  true  of  gold,  also  still  more  of  silver  and  platinum. 
It  is  a  property  which  belongs  in  a  high  degree  only  to  the 
native  metals  among  minerals. 

Sectile :  capable  of  being  cut  by  a  knife  like  cold  wax, 
so  that  a  shaving  may  be  turned  up  with  care,  and  yet  the 
mineral  breaks  with  a  blow,  and  is  not  properly  malleable, 
sometimes  not  at  all  so.  Cerargyrite  is  eminently  sectile. 
Gypsum  and  a  number  of  soft  minerals  are  imperfectly 
sectile.  No  sharp  line  separates  the  minerals  which  show 
these  characters  and  the  truly  brittle  minerals. 

Flexible :  the  mineral  bends  easily,  and  stays  bent  after 
the  pressure  is  removed.  This  is  shown  in  talc. 

Brittle :  separating  into  fragments  with  a  blow  or  with 
a  cut  by  a  knife.  This  is  true,  in  varying  degrees,  of 
nearly  all  minerals. 

The  elasticity  is  another  physical  character  based  upon 
cohesion.  A  mineral  is  said  to  be 

Mastic  when  it  is  capable  of  being  bent  or  pulled  out  of 
shape  and  then  returning  to  its  original  form  when  re- 
lieved, as  a  plate  of  mica.  Oil  the  other  hand,  a  cleavage- 
plate  of  chlorite  is  inelastic. 


THE   OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     79 

2.  SPECIFIC  GRAVITY  OR  RELATIVE  DENSITY. 

It  has  been  shown  that  the  eye-examination  of  a  mineral 
tells  the  observer  something  about  its  form,  if  crystallized, 
and  of  its  structure  in  other  cases;  it  tells  whether  it  has 
cleavage  or  not,  and  with  what  kind  of  surface  it  breaks; 
a  simple  trial  also  shows  how  hard  it  is.  At  the  same 
tim -,  as  it  rests  in  the  hand,  it  should  be  noted  whether  it 
seems  distinctly  heavy  or  light  as  compared  with  some 
common  substances  of  similar  appearance.  In  this  way  a 
first  suggestion  is  obtained, — not  exact  and,  as  we  shall 
see,  not  always  correct — as  to  the  density  of  the  mineral. 

It  is  necessary  at  the  outset  to  have  a  pretty  clear  notion 
as  to  what  a  difference  in  density  means.  Suppose  two 
doors  of  just  the  same  size,  and  both  swung  carefully  on 
hinges  so  that  they  move  almost  without  friction,  but  one 
of  wood,  the  other  of  iron.  Every  one  knows  that  the  force 
required  to  push  the  iron  door  will  be  much  greater  than 
that  which  the  other  requires.  Again,  a  kick  against  a 
wooden  ball  resting  on  the  ground  encounters  more  resist- 
ance than  against  a  ball  of  paper,  but  less  than  one  of 
stone.  These  experiments  show  that  in  a  piece  of  iron 
there  is  more  matter  to  move, — it  has  a  greater  mass  for 
the  same  bulk  than  the  wood.  In  other  words,  it  has  a 
greater  density.  This  is  generally  expressed  by  saying 
that  the  weight  of  the  iron  is  greater  than  of  the  wood  of 
the  same  size,  and  this  is  true  because  the  attraction  of  the 
earth,  which  gives  an  iron  ball  its  weight,  is  in  proportion 
to  the  mass  or,  for  different  bodies  of  the  same  size,  to  the 
density. 


80  MINERALS,  AND   HOW   TO   STUDY   THEM. 

We  may  say,  consequently,  that  the  relation  of  the  den- 
sities of  different  bodies  is  given  by  the  weights  of  blocks 
having  the  same  bulk.  Suppose  we  could  cut  out  like 
blocks  of  aluminium  and  iron  and  weigh  them,  the  weights 
would  be  not  far  from  the  ratio  of  1  to  3,  and  this  would 
be  the  relation  in  density.  Now  to  make  this  comparison 
for  all  bodies,  it  is  evidently  important,  in  the  first  place, 
to  choose  some  one  of  them  as  the  standard,  and  make  the 
comparison  with  this. 

The  standard  substance  adopted  is  pure  water,  and,  if 
great  accuracy  is  required,  this  must  be  taken  at  the  tem- 
perature 39°.2  F.  (4°  Centigrade),  where  it  has  its  maxi- 
mum density;  for  water,  if  growing  cooler  or  warmer  than 
39°.2  F.,  expands  a  little  and  grows  less  dense.  The  den- 
sity of  minerals  is  then  compared  with  water,  and  this 
density  is  called  the  specific  gravity.  Consequently  the 
specific  gravity  of  a  mineral  may  be  stated  to  be  the  weight 
of  a  fragment  divided  by  that  of  an  equal  volume  of  water. 
The  specific  gravity  of  sulphur  is  2,  of  corundum  4,  of 
pyrite  5,  etc.,  these  numbers  meaning  that  they  are  re- 
spectively 2  times,  4  times,  and  5  times  as  dense  as  water, 
or,  in  other  words,  a  given  bulk — a  cubic  foot,  for  example 
— weighs  2,  4,  and  5  times  more  than  the  same  bulk — a 
cubic  foot — of  water. 

In  order  to  find  the  specific  gravity,  it  is  not  practicable 
to  compare  at  once  the  weights  of  equal  volumes,  simply 
because  (though  it  is  easy  to  weigh,  for  example,  a  piece  of 
calcite)  it  is  not  possible  to  get  its  volume  with  sufficient 
accuracy.  Hence  it  is  necessary  to  make  use  of  a  well- 
known  principle  in  hydrostatics,  that  when  a  body  is  im- 


THE   OTHER   PHYSICAL  CHARACTERS  OF   MINERALS.     81 

mersed  in  a  liquid  it  is  buoyed  up  by  the  liquid,  and  to 
such  an  extent  that  it  weighs,  for  example  in  water,  less 
than  it  does  in  the  air  by  the  weight  of  the  water  it  dis- 
places. Hence,  if  we  find  first  the  weight  of  the  fragment 
on  the  pan  of  a  delicate  balance,  and  then  its  weight  in  the 
water,  it  being  suspended  from  the  pan  by  a  fine  thread, 
and  subtract  the  two  weights,  the  difference  is  the  weight 
of  the  equal  voh;me  of  water. 

For  example,  the  weight  of  a  little  quartz  crystal  is 
3.455  grams  in  the  air;  in  the  water  it  is  2.156;  the  loss 
of  weight,  or  weight  of  a  volume  of  water  exactly  equal  to 
it,  is  therefore  1.299;  hence  the  specific  gravity  is 

3.455  3.455 


3.455  -  2.156       1.299 

In  the  description  of  species  the  specific  gravity  is  often 
expressed  by  the  initial  letter  G;  thus  for  a  quartz  crystal 
G  =  2.66. 

A  spring-balance*  like  that  of  Jolly  makes  the  operation 
very  easy.  This  consists  of  a  delicate  brass  spring,  one  end 
of  which  is  attached  to  the  top  of  a  vertical  scale,  and  from 
the  other  hang  two  pans,  the  lower  one  of  which  is  im- 
mersed in  water.  The  small  fragment  whose  specific 
gravity  is  to  be  determined  is  placed  in  the  upper  pan,  and 
the  amount  that  the  spring  is  stretched  noted  by  the  num- 
ber (JV,)  coinciding  with  the  reflection  of  an  index  in  a 
strip  of  mirror  upon  which  the  scale  is  graduated ;  then  in 
the  lower  pan,  and  the  scale  number  (N9)  again  noted; 
the  number  (n)  when  the  pans  are  empty  is  also  ob- 

*  This  is  figured  on  page  69  of  Dana's  Manual  of  Mineralogy,  1887. 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


served.     The  specific  gravity  is  then  given   by  the  ex- 
pression 


In  this  case  we  do  not  have  the  actual  weights  given,  but 
numbers  which  are  proportional  to  them. 

A  simple  balance  for  de- 
termining the  specific  grav- 
ity, which   also  does   away 
with  the  necessity  of  using 
definite  weights,  is  shown  in 
3    Fig.  147  (one  sixth  natural 
I    size),  and  can  easily  be  cou- 
|    structed  *  with  a  little  care. 
|    After    the    frame    is   made 
>,    with    the    two    uprights,   a 
g    thin  piece  of  wood  is  cut  in 
«    the    form  of   the  steelyard 
|    beam,  ale.     This  should  be 
g    graduated  into  inchesf  and 
|    tenths  of  inches  from  b,  the 
§    axis,  to  the  end  c;  the  dis- 
tance from  5  to  a,  where  the 
pans   are    supported,    being 
made  4  inches.     A  fine  wire 
passed  through  at  b  answers 
as  the  axis,  and  the  pans  are 

*  The  author  is  indebted  to  Prof.  Penfield  for  the  opportunity  to 
figure  and  describe  this  balance. 

f  The  metric  scale  may  be  conveniently  used  instead. 


THE   OTHER   PHYSICAL   CHARACTERS  OF   MINERALS.     83 

also  held  by  wires,  best  of  platinum;  the  lower  pan  is  im- 
mersed in  water.  A  small  piece  of  lead  permanently  placed 
between  b  and  a  serves  to  counterpoise  the  long  arm,  and 
a  little  rider  is  first  moved  to  some  point,  as  d,  where  it 
serves  to  make  be  exactly  horizontal,  as  shown  by  the 
mark  on  the  upright  near  the  end  c.  A  number  of  weights 
(as  from  £  to  If  grams)  are  further  needed;  these  may  be 
easily  made  of  soft  copper  wire  bent  with  a  hook  at  one 
end,  or  they  may  be  little  glass  tubes  containing  1,  2,  3  or 
more  shot  and  with  a  wire  hook  fused  in  at  one  end  (see 
those  at  g).  The  fragment*  experimented  upon  is  first 
placed  in  the  pan  e,  a  suitable  counterpoise  chosen,  and  the 
number  (JV,)  on  the  scale  where  it  must  be  placed  to  make 
be  again  horizontal  recorded;  it  is  then  transferred  to  the 
pan  /,  immersed  in  the  water,  and  the  scale  number  again 
noted  (N9),  the  same  counterpoise  being  employed.  The 
first  number  then  divided  by  the  difference  of  the  two 
numbers  gives  the  specific  gravity: 


For  example,  a  small  pyrite  crystal  is  placed  in  the  pan  e> 
and  a  counterpoise  chosen  which  balances  the  beam  when 
placed  14.45  inches  from  b;  the  crystal  is  then  transferred 

*  A  fragment  or  crystal  as  heavy  as  7  grams  may  be  employed 
with  the  weights  mentioned;  but  it  should  be  understood  that  it  is 
not  necessary  to  know  the  actual  weight  either  of  the  fragment  or  of 
the  counterpoise;  thus  if  the  actual  weight  of  the  counterpoise  is  C, 
and  of  the  mineral  in  air  and  in  water  Wi  and  TF»  respectively,  then, 
using  the  numbers  Ni  and  N*  as  above,  we  shall  have 

TF,  X  4  =  G  X  N-     and     TT.X4  =  (7x^i;    that  is,  ~  =  ~. 


84  MINERALS,  AND   HOW  TO   STUDY  THEM. 

to  the  pan  /,  which  requires  that  this  counterpoise  should 
be  moved  back  to  11.55  inches.  The  specific  gravity  is 
thus  found  to  be 

14.45  14.45 


14.45  —  11.55         2.9 


=  4.98. 


It  is  evident  that  the  accurate  determination  of  the 
ipecific  gravity  is  a  somewhat  difficult  matter,  requiring  a 
£ood  deal  of  care,  but,  as  was  suggested  at  the  "beginning, 
the  hand  can  often  give  very  valuable  information  in  this 
direction  after  a  little  training.  We  are  accustomed  to 
handle  fragments  of  rocks,  as  of  granite  (specific  gravity 
about  2.7),  of  marble  (specific  gravity  =  2.7),  and  of  other 
like  substances,  and  we  know  about  what  to  expect  as  to 
the  weight  of  them;  hence  if  we  pick  up  a  piece  of  barite 
or  heavy  spar,  perhaps  thinking  it  is  marble,  which  it  may 
resemble  so  closely  that  the  unaided  eye  cannot  distinguish 
them,  we  notice  or  ought  to  notice  at  once  that  it  is  un- 
expectedly heavy,  for  its  specific  gravity  is  4.5.  So  a  piece 
of  corundum,  we  say,  feels  heavy  because  it  has  a  specific 
gravity  as  high  as  4.  This  last,  by  the  way,  is  an  interesting 
case,  because  corundum  is  the  oxide  of  the  very  light  metal 
aluminium,  and  its  relatively  high  density  is  connected  with 
its  great  hardness;  in  other  words,  it  is  evident  that  its 
molecules  must  be  much  crowded  together.  In  the  same 
way  we  note  that  the  substance  carbon  forms  the  hard  and 
relatively  heavy  *  mineral  diamond  (specific  gravity  =  3.5) 

*  In  these  statements,  as  in  some  similar  cases,  the  word  heavy  ia 
used  instead  of  dense,  that  is,  of  high  specific  gravity,  and  also  the 
word  light  to  express  the  opposite  character;  the  meaning  will  b« 
clear  even  if  these  terms  are  not  quite  scientifically  employed. 


THE    OTHER    PHYSICAL    CHARACTERS    OF    MINERALS.      85 

and  the  soft  and  light  mineral  graphite  (specific  gravity  = 
2).  In  other  words,  the  density  depends  not  only  upon  the 
kind  of  molecules  (all  lead  compounds,  for  example,  have 
necessarily  a  high  specific  gravity),  but  also  upon  the  way 
they  are  built  up  together. 

Again,  if  we  take  up  a  metallic  mineral,  we  compare  it, 
perhaps  without  conscious  thought,  with  other  common 
metallic  substances.  Hence  a  piece  of  aluminium  seems 
very  light,  because  the  specific  gravity  is  only  2.5,  about 
one  third  that  of  iron  and  less  than  one  quarter  that  of 
silver.  On  the  other  hand,  a  fragment  of  galena  seems 
heavy  because  its  specific  gravity  is  7.5,  or  nearly  equal  to 
that  of  metallic  iron. 

When  we  come  to  study  minerals  we  find  that  they 
divide  themselves  as  follows: 

I.  Minerals  of  Unmetallic  Luster* — These  may  be 
roughly  subdivided  into  three  classes : 

1.  Minerals  of  relatively  low  density;  the  specific  gravity 
not  higher  than  2.5.     Examples  are  as  follows  (here  G.  = 

specific  gravity): 

G.  G. 

Borax 1.7          Stilbite 2.2 

Sulphur 2.05         Gypsum 2. 3 

Halite  2.1          Apophyllite  2.4 

The  zeolites  (as  stilbite  above)  mostly  fall  between 
G.  =  2.0  and  G.  =  2.3. 

2.  Minerals  of  average  density:  specific  gravity  2.6  to  3. 
Common  examples  are : 

*  This  matter  of  luster  is  more  definitely  spoken  of  on  a  later  page 
of  this  chapter. 


86  MINERALS,  AND   HOW   TO   STUDY   THEM. 

G.  G. 

Quartz.. 2.66        Feldspar 2.6-2.75 

Beryl 2.7          Talc 2.8 

Calcite 2.7          Muscovite 2.8 

The  scapolites  (G.  =  2.5-2.8)  also  belong  in  this  group. 

The  common  minerals  tourmaline  (G.  =  3.0-3.2),  apatite 
(G.=  3.2),  vesuvianite  (G.=  3.4),  amphibole  (G.=  2.9-3.4), 
pyroxene  (G.=  3.2-3.6),  epidote  (G.=  3.25-3.5)  fall  between 
this  and  the  following  group.  Some  varieties  of  garnet 
also  belong  here;  others  have  a  specific  gravity  up  to  4.3. 

3.  Minerals  of  high  density :  specific  gravity  3.5  or  above; 
a  fragment  seems  heavy  in  the  hand;  if  the  specific  gravity 
is  above  4.5,  it  seems  very  heavy. 

G.  G. 

Topaz 3.5          Witherite 4.3 

Diamond 3.52        Barite 4.5 

Staurolite 3.7          Zircon 4.7 

Strontianite 3.7          Scheelite 6.0 

Celestite 3.96        Cassiterite 7.0 

Corundum 4.0          Wolframite 7.5 

Eutile 4.2          Cinnabar 8.0 

The  compounds  of  lead  also  belong  here,  the  commonest  of 
which  are:  cerussite,  the  carbonate;  anglesite,  the  sulphate; 
and  pyromorphite,  the  phosphate.  They  all  have  a  specific 
gravity  between  6  and  7.  Compounds  of  iron  (as  siderite, 
G.  =  3.8),  of  copper  (as  cuprite,  G.  =  6.0),  of  silver  (as 
cerargyrite,  G.  =  5.5),  and  of  the  other  heavy  metals  have 
also  high  specific  gravities. 


THE   OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     87 

II.  Minerals  with  Metallic  Luster. — The  average  is 
about  5,  the  specific  gravity  of  pyrite  and  hematite.  If 
the  density  is  much  lower  than  4,  the  mineral  seems  light 
in  the  hand,  as  graphite  (G.  =  2).  If  the  density  is  as  high 
as  7,  or  above,  they  seem  notably  heavy,  as  galena,  7.5. 
Crystallized  uraninite,  which  has  a  submetallic  luster,  has 
the  remarkably  high  specific  gravity  of  9.7  when  unal- 
tered. The  variation  is  wider  in  the  case  of  the  metallic 
minerals  than  with  those  of  un metallic  luster,  as  vill  be 
seen  by  comparing  the  following  densities  of  the  common 
metals  :* 


Aluminium 2.5 

Arsenic 5.7 

Antimony 6.7 

Zinc 7.1 

Tin 7.3 

Iron 7.8 

Copper  8.9 


Bismuth 9.8 

Silver 10.6 

Lead 11.4 

Mercury  (liquid) 13.6 

Gold 19.3 

Platinum  (pure) 21.5 


The  quick  judgment  which  comes  with  practice  is  al- 
ways of  value,  but  it  should  be  applied  with  discretion,  for 
the  mineralogist  must  be  continually  on  his  guard  lest  he 
be  misled. 

In  the  first  place,  the  size  of  the  mass  is  an  important 
factor,  for  a  big  lump  of  quartz  seems  heavy,  of  course, 
though  its  specific  gravity  is  not  relatively  high.  Also  we 

*  It  is  interesting  to  add  the  following,  although  these  metals  do 
not  exist  as  such  in  nature,  and  are  only  interesting  to  the  chemist : 

Lithium 0.59        Sodium 0.97 

Potassium 0.86        Magnesium 1.8 


88  MINERALS,  AND   HOW   TO   STUDY   THEM. 

may  get  a  wrong  impression  in  handling  a  specimen  if  the 
mineral  we  are  interested  in  only  forms  a  small  part  of  it; 
a  little  galena  in  a  large  mass  of  quartz  will  not  make  it 
heavy.  Also,  if  the  mineral  is  open  and  porous,  and  is 
made  up  of  interlacing  fibers,  like  some  specimens  of  ce- 
russite,  it  may  appear  light,  even  if  the  specific  gravity  is 
actually  high,  because  the  eye  is  deceived  by  the  appear- 
ance of  bulk,  while  the  solid  mass  present  is  not  great. 
Some  further  suggestions  on  this  subject  are  given  in  the 
closing  chapter  of  this  book. 

3.  CHARACTERS  DEPENDING  UPON  LIGHT. 

Of  the  characters  which  are  observed  by  the  eye  several 
have  not  been  mentioned  in  detail  as  yet,  to  which  perhaps 
the  attention  may  be  first  attracted.  These  are  those 
which  depend  upon  the  reflection  or  absorption  of  the 
light:  (1)  the  luster  or  the  appearance  of  the  surface  in- 
dependent of  the  color,  due  to  the  way  the  light  is 
reflected;  (2)  the  color;  and  (3)  the  degree  of  trans- 
parency. 

Luster. 

The  difference  in  luster  is  not  in  all  cases  easy  to  de- 
scribe, but  the  eye  notes  it  at  once,  and  after  a  little  train- 
ing seldom  makes  a  mistake. 

The  kinds  of  luster  distinguished  are  as  follows: 

Metallic :  the  luster  of  a  metallic  surface  as  of  steel,  lead, 

tin,  copper,  gold,  etc.   This  is  not  always  easy  to  distinguish, 

and  the  rule  is  an  important  one  that  the  luster  is  not  called 

metallic  unless  the  mineral  is  quite  opaque,  so  that  no  light 


THE   OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     89 

passes  through  even  very  thin  edges.  The  luster  of  some 
minerals,  as  columbite,  is  said  to  be  submetallic  when  it 
lacks  the  full  luster  of  the  metals.  In  a  few  cases  a  min- 
eral has  varieties  with  metallic  and  others  with  uninetallic 
luster;  this  is  true  of  hematite. 

Vitreous,  or  glassy  luster:  that  of  a  piece  of  broken 
glass.  This  is  the  luster  of  most  quartz  and  of  a  large  part 
of  non-metallic  minerals. 

Adamantine,  or  the  luster  of  the  diamond:  this  is  the 
brilliant,  almost  oily,  luster  shown  by  some  very  hard  min- 
erals, as  diamond,  corundum,  etc.;  also  of  some  others, 
having  heavy  molecules,  as  the  carbonate  (cerussite)  and 
the  sulphate  (anglesite)  of  lead.  All  of  these  refract  the 
light  strongly,  or  have  a  high  refractive  index. 

Metallic-adamantine  is  a  term  used  to  describe  a  variety 
of  the  adamantine  luster  verging  upon  metallic,  as  seen  in 
some  dark-colored  varieties  of  cerussite. 

Resinous  or  waxy :  the  luster  of  a  piece  of  rosin,  as  that 
of  most  kinds  of  sphalerite  or  zinc  blende;  near  this,  but 
often  quite  distinct,  is  greasy  luster,  shown  by  some  speci- 
mens of  milky  quartz. 

Pearly,  or  the  luster  of  mother-of-pearl:  this  is  common 
where  a  mineral  has  very  perfect  cleavage  and  hence  has 
partially  separated  into  thin  plates.  Thus  the  basal  or  top 
plane  of  crystals  of  apophyllite  shows  pearly  luster. 

Silky,  the  luster  of  a  skein  of  silk  or  a  piece  of  satin : 
this  is  characteristic  of  some  minerals  with  fibrous  struc- 
tures, as  the  variety  of  calcite  (or  of  gypsum)  called  satin 
spar;  also  of  most  asbestus. 

The  luster  of  minerals  is  also  described  according  to  the 


90  MINERALS,  AND   HOW   TO    STUDY   THEM. 

brightness  of  the  surface;  it  is  called  splendent  in  freshly 
fractured  galena,  but  dull  in  jasper;  while  again  it  may  be 
glistening  or  glimmering  according  to  the  nature  of  the 
surface.  These  terms  explain  themselves. 

Color. 

To  understand  what  the  color  of  a  mineral  means  a  little 
knowledge  of  optics  is  required.  In  the  first  place,  W3 
must  recall  that  ordinary  light  can  be  separated,  as  by  a 
glass  prism,  into  the  ribbon  of  colors  ranging  from  the  red 
to  the  blue  and  violet  which  is  called  the  spectrum.  This 
is  shown  in  the  rainbow,  where  the  place  of  the  prism  is 
taken  by  the  raindrops.  All  these  different  colors  together 
give  to  the  eye  the  effect  of  white  light.  If  now  this 
white  light,  as  the  ordinary  sunlight,  be  passed  through  a 
piece  of  red  glass  or  reflected  from  a  surface  of  red  paint, 
part  of  the  colors  are  stopped  or  absorbed  by  the  glass  or 
paint  and  the  rest  give  together  the  effect  of  red  which 
the  eye  notes.  Similarly  a  piece  of  malachite  appears 
green  for  the  same  reason  that  the  grass  does,  because  the 
surface  absorbs  a  part  of  the  light,  and  the  remainder 
which  reaches  the  eye  gives  to  it  the  effect  of  green. 

The  variation  in  color  is  very  wide  and  includes  all 
kinds  from  white  to  black,  running  through  many  shades 
of  red,  yellow,  green,  and  b'lue.  Most  of  the  terms  used 
in  describing  the  color  are  so  familiar  that  they  explain 
themselves.  Thus  we  speak  of  azure-blue,  cherry-red,  and 
so  on.  Some  examples  of  color  among  common  minerals 
are  given  in  Chapter  VIII. 


THE   OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     91 

Now  a  few  words  as  to  the  importance  of  color  and  the 
extent  to  which  it  may  vary  with  the  same  species. 

As  we  go  on  to  study  the  different  mineral  species  we 
learn  that  the  varieties  of  those  with  metallic  luster  do 
not  vary  much  in  color  among  themselves.  Many  of  the 
other  minerals,  however,  which  do  not  have  a  metallic 
luster  vary  very  widely;  and  in  the  same  species,  as  tourma- 
line, we  may  have  colorless  kinds,  also  those  that  are  differ- 
ent shades  of  red,  blue,  green,  brown,  and  black. 

So  too  we  call  clear  red  kinds  of  corundum  ruby,  and 
clear  blue  kinds  sapphire,  although  they  both  belong  to 
the  same  mineral;  and  in  this  as  in  many  other  cases  it  is 
difficult  to  tell  precisely  what  is  the  very  slight  chemical 
change  upon  which  the  color  depends. 

In  general,  therefore,  the  color  of  minerals  with  metallic 
luster  is  a  very  important  and  constant  character,  and  such 
variations  as  are  noted  are  due  chiefly  to  a  little  change  of 
the  surface,  by  which  it  becomes  tarnished,  as  we  say;  that 
is,  dull  in  some  cases,  or  perhaps  bright-colored  in  others. 
Thus  galena  has  a  very  characteristic  bright  bluish  lead- 
gray  color,  which,  however,  may  become  quite  dull  if  the 
surface  has  been  long  exposed  to  the  air.  Again,  the  pale 
brass-yellow  of  pyrite  and  the  still  paler  shade  of  the 
same  color  characteristic  of  fresh  marcasite  are  readily  dis- 
tinguished, but  both  are  subject  to  tarnish,  especially  the 
latter,  and  this  may  result  in  making  the  distinction  be- 
tween the  two  impossible  in  the  case  of  exposed  surfaces. 

Hence  it  is  necessary  to  be  on  our  guard  to  insure  that 
what  we  see  is  the  true  color  of  a  mineral,  that  is,  the 
color  which  is  obtained  from  a  fresh  surface  by  fracture. 


92  MINERALS,  AND   HOW   TO   STUDY  THEM. 

Another  striking  example  is  given  by  the  mineral  bornite, 
which  has  a  very  peculiar  reddish-bronze  color  on  a  fresh 
surface,  on  account  of  which  the  miners  of  Cornwall  have 
called  it  horse-flesh  ore.  But  this  changes  or  tarnishes  so 
easily  that  it  almost  immediately  becomes  colored  bluish, 
purple,  etc.,  the  color  depending  on  the  time  it  has  been 
exposed.  Because  of  this  character  the  mineral  is  some- 
times called  purple  copper  ore  or  variegated  copper  ore. 
So  too  chalcopyrite,  which  is  bright  brass-yellow  when 
fresh,  may  become  iridescent  as  in  the  variety  called  pea- 
cock ore — a  name  which,  by  the  way,  is  also  given  to  bor- 
nite. 

It  is  often  important,  especially  with  a  mineral  having 
metallic  luster,  to  test  the  color  of  the  fine  powder,  or 
the  color  of  the  streak.  The  slight  scratch  which  is  given 
to  test  the  hardness  will  often  show  this,  but  a  better  way 
is  to  have  at  hand  a  piece  of  rough  white  porcelain,  or  one 
of  ground  glass  upon  which  the  mineral  can  be  rubbed. 
This  method  shows,  for  example,  that  hematite,  sometimes 
iron-black  in  color  and  with  a  bright  metallic  luster,  has 
a  red  streak;  this  is  indeed  so  important  a  character  that 
it  is  the  source  of  the  mineral's  name  (from  the  Greek 
capa,  meaning  blood). 

Minerals  of  unmetallic  luster  usually  have  a  streak 
which  differs  but  little  from  white  even  if  the  mineral 
itself  is  dark-colored  or  even  black;  this  is  true,  for  exam- 
ple, of  the  different  varieties  of  tourmaline. 

Transparency. 
A  mineral  is  said  to  be  transparent  when  it  is  so  clear 


THE   OTHER   PHYSICAL  CHARACTERS  OF   MINERALS.     93 

that  an  object  can  be  seen  through  it  with  perfect  distinct- 
ness, as  a  piece  of  window-glass,  a  plate  of  selenite,  or  a 
thin  sheet  of  mica.  It  is  semi-transparent  or  subtranspa- 
rent  when  the  outlines  of  an  object  can  be  seen,  but  not 
distinctly. 

A  mineral  is  translucent  when  it  transmits  light,  as  a 
piece  of  thin  porcelain,  but  does  not  allow  an  object  to  be 
seen  through  it.  It  is  subtranslucent  when  light  is  trans- 
mitted only  on  the  edges. 

When  describing  the  color  of  a  mineral,  some  peculiar- 
ities in  its  distribution  may  be  noted  and  receive  special 
names.  A  mineral  is  said  to  show  a  play  of  colors  when, 
like  the  opal,  it  exhibits  internally  the  various  prismatic 
colors  when  the  mineral  is  turned. 

A  pearly  reflection  from  the  interior  of  a  mineral,  like 
the  effect  of  a  glass  of  water  to  which  a  few  drops  of  milk 
have  been  added,  is  called  opalescence  because  common 
with  the  opal. 

Iridescence  is  the  presenting  of  prismatic  colors  on  the 
surface  of  a  specimen,  and  then  usually  due  to  tarnish;  or 
in  the  interior  of  a  mineral,  and  then  often  explained  by 
the  presence  of  thin  twinning  lamella  or  of  minute  air- 
spaces, as  in  a  cleavage  mass  of  calcite. 

Asterism  is  the  name  given  to  the  peculiar  starlike 
effect  seen  sometimes  by  reflected  light,  as  in  certain 
kinds  of  sapphire;  or  by  transmitted  light,  as  in  some 
mica  when  a  candle-flame  is  viewed  through  it.  It  is  due 
in  the  first  case  to  planes  of  structure  intersecting  symmet- 
rically in  the  crystal;  in  the  second  to  the  presence  of 


94 


MINERALS,  AND   HOW   TO   STUDY    THEM. 


minute  crystals  of  a  foreign  mineral  (often  rutile)  symmet 
rically  arranged  between  the  plates  of  the  mica. 

Phosphorescence  is  the  property  of  becoming  luminous 
when  slightly  heated,  as  shown  by  fluorite,  especially  the 
variety  chlorophane;  also  upon  friction,  as  some  marble, 
tremolite,  etc.;  again,  when  exposed  to  the  sun -rays  or  to 
an  electric  discharge,  this  last  being  especially  true  of  some 
of  the  gems. 

Optical  Characters.— Only  a  few  words  can  be  devoted 
here  to  the  large  and  important  class  of  optical  characters 
of  minerals,  depending  upon  the  action  of  light  on  them 
as  determined  by  their  molecular  structure.  The  under- 
standing of  this  part  of  Physical  Mineralogy  demands  first 
a  good  knowledge  of  crystallography,  and  further  a  mastery 
of  optics,  especially  of  the  difficult  subject  of  polarization. 
Only  one  or  two  points  belonging  here  can  be  touched 
upon. 

148. 


Iceland  Spar  or  Double  refracting  Spar. 

One  of  these  is  double  refraction,  or  the  separation  of  a 
ray  of  light  passing  through  certain  crystalline  substances 


THE    OTHER   PHYSICAL  CHARACTERS  OF   MINERALS.     95 

into  two  rays.  This  is  indeed  true,  in  general,  of  all  trans- 
parent crystals,  except  those  of  the  isometric  system,  but 
the  only  mineral  in  which  it  is  noted  to  a  marked  degree  is 
calcite,  especially  in  the  transparent  variety  called  Iceland 
spar.  Fig.  148  illustrates  this  property  well;  there  the  single 
cross  on  the  paper  beneath  appears  double  to  the  eye;  one 
cross  (to  the  eye  looking  perpendicularly  down  on  the  sur- 
face) has  its  arms  in  the  continuation  of  the  lines  beneath, 
the  other  is  pushed  to  one  side.  Neither  cross  appears 
quite  black  except  at  the  two  points  where  they  intersect. 

Another  point  in  this  connection  is  the  dichroism  of  a 
crystal,  or  the  appearance  of  different  color,  as  it  is 
viewed  by  transmitted  light  in  different  directions;  this  is 
due  to  varying  degrees  of  light  absorption.  This  is  often 
seen  in  transparent  crystals  of  epidote;  it  is  also  to  this 
cause  that  the  different  appearance  of  a  crystal  of  musco- 
vite  is  due,  first,  in  the  direction  through  it  and  again  at 
right  angles  to  the  cleavage. 

4.  CHARACTERS  DEPENDING  UPON  HEAT. 

The  fusibility  of  minerals,  or  their  relative  power  of  being 
melted  at  a  more  or  less  elevated  temperature,  is  the  most 
important  character  depending  upon  the  action  of  heat. 
This  is  discussed  in  another  place  in  connection  with 
the  description  of  the  use  of  the  blowpipe.  The  con- 
ductivity of  crystals  for  heat  is  another  point  which  some- 
times is  experimented  upon.  As  would  be  expected,  it  de- 
pends upon  their  molecular  structure  in  different  directions. 
This  and  other  related  subjects  belong  to  advanced  miner- 
alogy. 


96  MINERALS,  AND   HOW   TO    STUDY   THEM. 


5.  CHARACTERS  DEPENDING  UPON  MAGNETISM. 

A  few  minerals  have  the  property  of  being  attracted  by  a 
magnet.  This  is  true  of  magnetite,  or  the  magnetic  oxide 
of  iron;  of  pyrrhotite,  or  magnetic  pyrites;  also  of  some 
specimens  of  native  platinum. 

A  specimen  of  magnetite  sometimes  is  itself  a  magnet, 
and  has  then  the  power  of  attracting  little  particles  of 
iron  or  steel;  it  has  a  north  and  south  pole,  and  if  hung  by 
a  thread  will  swing  around  until  the  poles  oome  into  the 
magnetic  meridian,  that  is,  the  direction  assumed  by  a 
compass-needle.  This  kind  of  magnetite  is  called  the  lode- 
stone.  Pyrrhotite  is  much  less  strongly  magnetic  than 
magnetite,  and  the  magnetic  varieties  of  platinum  are  not 
common;  both  may  have  polarity  like  the  lodestone.  A 
few  minerals,  as  hematite  and  franklinite,  are  sometimes 
slightly  magnetic,  but  probably  only  because  they  contain 
a  little  admixed  magnetite. 

Most  minerals  containing  much  iron  become  magnetic 
when  heated  in  the  reducing-flame  of  the  blowpipe;  this 
is  true  also  of  millerite  or  the  sulphide  of  nickel. 

6.  CHARACTERS  DEPENDING  UPON  ELECTRICITY. 

There  are  a  number  of  electrical  properties  of  minerals, 
but  these  are  characters  that  belong  to  a  more  minute 
study  of  minerals,  and  they  need  be  only  briefly  mentioned 
here. 

A  number  of  minerals,  like  sulphur,  the  diamond,  and 


THE    OTHER   PHYSICAL   CHARACTERS   OF   MINERALS.     97 

topaz,  become  rather  strongly  electric  when  rubbed,  as  with 
a  piece  of  silk,  and  show  this  by  their  power  of  attracting 
light  substances,  such  as  bits  of  straw  or  paper.  Again, 
the  crystals  of.  some  minerals,  when  carefully  heated  or 
cooled,  become  electrified  and  show  opposite  kinds  of  elec- 
tricity in  different  parts,  as  at  the  two  extremities;  this  is 
particularly  true  of  tourmaline.  It  is  remarked  on  p.  318 
that  when  both  ends  of  a  tourmaline  crystal  are  developed 
it  is  common  to  find  them  different  in  their  crystalline 
faces.  This  dissimilarity  in  structure  in  the  opposite  direc- 
tions of  the  axis  is  connected  with  the  property  of  becom- 
ing dissimilarly  electrified  by  change  of  temperature.  This 
subject  is  called  pyro-electricity,  because  the  electrical  effect 
is  due  to  the  action  of  heat  (nvp,  fire).  Tourmaline  is 
hence  said  to  be  pyro-electric,  and  the  same  is  true  of 
quartz,  and  less  strikingly  of  many  other  species. 

7.  TASTE  AND  ODOR. 

Taste  belongs  only  to  the  few  minerals  which  dissolve  to 
some  extent  in  water.  The  terms  employed  are  familiar 
and  hardly  need  explanation.  Saline  means  the  taste  of 
common  salt;  alkaline,  of  soda;  bitter,  of  epsom  salts;  sour, 
of  an  acid;  astringent, of  iron  vitriol;  sweetish  astringent, 
of  alum;  cooling,  of  saltpeter. 

Odor  also  belongs  to  a  few  minerals  only.  Some  va- 
rieties of  limestone,  barite,  or  quartz  have  a  fetid  odor, 
or  odor  of  rotten  eggs,  especially  if  rubbed  sharply;  this 
is  usually  due  to  the  presence  of  some  sulphur  compound. 
Moistened  clay  and  some  claylike  minerals  when  breathed 


98  MINERALS,  AND    HOW   TO    STUDY   THEM. 

upon  give  off  a  peculiar  argillaceous  odor.     Bitumen  and 
some  allied  substances  have  a  bituminous  odor. 

A  sharp  blow  across  the  surface  of  a  piece  of  arseno- 
pyrite  often  produces  a  peculiar  garlic  odor,  like  that 
obtained  by  heating  the  same  mineral  on  charcoal,  and  in 
fact  due  to  the  same  cause.  Similarly  a  blow  on  a  mass  of 
pyrite  may  yield  a  sulphurous  odor 


THE   CHEMICAL   CHARACTERS   OF   MINERALS.  99 


CHAPTER  V. 
THE  CHEMICAL  CHARACTERS  OF  MINERALS. 

IT  has  already  been  stated  that  every  mineral  is  neces- 
sarily a  definite  chemical  compound,  and  that  this  is  the 
most  essential  point  in  the  definition  of  a  mineral.  But 
to  understand  what  a  chemical  compound  is,  and  what  re- 
lations different  compounds  bear  to  each  other,  requires 
some  knowledge  of  the  fundamental  principles  of  chemis- 
try. In  the  first  place  it  is  necessary  to  understand  what 
the  chemical  elements  are. 

The  chief  work  of  the  chemist  in  the  laboratory  is  to 
analyze  different  substances,  or  in  other  words  to  separate 
them  into  the  various  kinds  of  matter  which  they  contain. 
But  this  process  of  analysis,  or  chemical  separation,  can 
only  be  carried  a  little  way,  for  the  chemist  soon  obtains 
substances  which  he  is  unable  to  decompose  further.  Thus 
if  he  takes  a  piece  of  calcite,  it  is  easy  by  simply  heating  it 
to  separate  it  into  a  white  powder  called  lime  (this  is  what 
the  mason  uses  for  making  mortar)  and  a  gas  called  car- 
bon dioxide,  or  carbonic-acid  gas.  Then  further,  if  the 
proper  means  are  taken,  the  lime  can  be  separated  into 
a  metal,  called  calcium,  and  a  gas,  oxygen;  while  the  car- 
bon dioxide  can  be  separated  into  the  familiar  substance 
carbon  and  the  same  gas,  oxygen.  But  these  three  sub- 
stances, calcium,  carbon,  oxygen,  cannot  be  decomposed 


100  MINERALS,  AND    HOW   TO   STUDY   THEM. 

further;  hence  they  are  called  elementary  substances  or 
elements.  Again,  common  salt  can  be  separated  into  two 
kinds  of  matter,  the  metal  sodium  and  the  gas  chlorine; 
but  neither  of  these  can  be  separated  any  further,  hence 
they  are  also  put  down  among  the  simple  or  elementary 
substances.  So,  too,  galena  can  be  separated  into  its  ele- 
ments, the  metal  lead  and  sulphur;  sugar  is  decomposed 
into  carbon  and  the  gases  hydrogen  and  oxygen;  and  many 
other  illustrations  might  be  given. 

These  substances,  then,  into  which. a  given  kind  of  mat- 
ter can  be  separated,  but  which  the  chemist  is  unable  to 
decompose  further,  are  the  CHEMICAL  ELEMENTS. 

Now  the  chemist  finds  that  although  there  is  no  limit 
to  the  different  kinds  of  bodies  which  he  may  be  asked 
to  analyze  or  separate  into  their  parts,  still  they  contain 
but  a  small  number  of  distinct  kinds  of  matter.  If  we  re- 
gard only  those  which  are  commonly  present,  they  are  very 
few  indeed.  There  are,  it  is  true,  about  seventy  of  the 
elements  recognized  by  the  chemist,  but  many  of  them  are 
excessively  rare,  and  those  which  make  up  the  chief  part 
of  common  minerals  are  hardly  more  than  twelve  or 
thirteen. 

The  table,  p.  101,  gives  the  names  of  all  the  common  ele- 
mentary substances  and  most  of  the  rarer  ones.  With  the 
names  are  given  also  the  initial  letter  or  letters  by  which 
they  are  generally  represented  in  the  kind  of  algebraic 
shorthand  that  the  chemist  employs;  these  letters  are 
called  the  symbols  of  the  elements.  Thus  oxygen  is  repre- 
sented by  the  capital  letter  0;  hydrogen  by  H;  nitrogen  by 
N;  calcium  by  Ca;  and  so  on.  In  a  good  many  cases  the 


THE    CHEMICAL    CHARACTERS   OF    MINERALS. 


101 


THE  CHEMICAL  ELEMENTS. 


Sym 
bol. 

Al 
Sb 
As 
Ba 
Be 
Bi 
B 
Br 
Cd 
Cs 
Ca 
C 
Ce 
Cl 
Cr 
Co 
um. 
Cu 
Di 
Er 
F 
Ge 
lor: 
Au 
H 
I 
Ir 
Fe 
La 
Pb 
Li 
Mg 
Mn 

Hg 

At. 
Weight. 

27 
120 
74.9 
137 
9.1 
207.5 
10.9 
79.8 
111.7 
132.8 
39.9 
12 
141 
35.5 
52.5 
58.7 

63.2 
142 
166 
19.1 
73.3 
Be    * 
196.7 
1 
126.5 
192.5 
55.9 
138 
2064 
7 
24 
54.8 

199.8 

Molybdenum  

Sym-      At. 
bol.  Weight. 

Mo      96 
Ni       58.6 
Nb      93.7 
N         14 
Os     191 
0         16 
Pd     106 
P         31 
Pt      194.3 
K         39 
Rh     104.1 
Rb       85.2 
Ru     103.5 
Sc        44 
Se        78.9 
Si         28 
Ag     107.7 
Na       23 
Sr        87.3 
S          32 
Ta      182 
Te     125 
Tl      203.7 
Th     232 
Sn      117.4 
Ti        48 

W      183.6 
U       240 
V         51.1 
Yt     172.6 
Y         89 
Zn       65.1 
Zr        90.4 

Antimony  (Stibium).  . 
Arsenic  

Nickel        

Niobium 

Barium     .        

Nitrogen 

Beryllium  .    . 

Osmium  

Bismuth 

Bromine  

Phosphorus  

Cadmium.  ....... 

Platinum  

CsBsium  

Potassium  (Kalium).  . 
Rhodium 

Calcium     .  . 

Carbon 

Chlorine  

Scandium  

Chromium  

Selenium    

Cobalt.  ... 

Silicon 

Columbium,  see  Niobi 
Copper  (Cuprum).  .  .  . 
Didymium  

Silver  (Argentum).  .  .  . 
Sodium  (Natrium).  .  . 
Strontium  

Erbium  

Sulphur  

Fluorine 

Tantalum  
Tellurium 

Germanium 

Thallium 

Hydrogen  

Tin  (Stannum)  
Titanium     

Iodine  

Iridium 

Tungsten  (Wolf  r  ami- 
urn) 

Iron  (Ferrum)  

Lead  (Plumbum)  
Lithium        .... 

Magnesium  

Yttrium  

Manganese  
Mercury     (Hydrargy- 
rum) 

Zinc     

Zirconium  

See  Beryllium. 


102  MINERALS,  AND   HOW   TO   STUDY   THEM. 

initial  letters  of  the  Latin  name  of  a  metal  are  used,  as  Fe, 
from  the  Latin  ferrum,  for  iron;  Ag,  from  argentum,  the 
Latin  name  of  silver;  Au,  from  aurum,  gold;  Sb,  from 
stibium,  antimony,  etc. 

The  numbers  placed  after  each  name  give  the  atomic 
weight  of  each  element.  What  this  is  will  be  explained 
immediately.  But  first  note  that  the  larger  part  of  the 
elements  are  metals,  having  physical  properties  of  luster, 
malleability,  etc.,  more  or  less  like  those  of  gold,  silve^ 
lead,  and  iron.  There  is  also  a  small  class  of  non-metals^ 
including  the  gases,  hydrogen,  oxygen,  etc.,  also  sulphur, 
phosphorus,  silicon,  and  carbon.  Further,  a  few  elements 
standing  between  the  two  groups  are  sometimes  called 
semi-metals,  as  tellurium,  arsenic,  antimony.  The  chemi- 
cal distinction  between  the  metal  and  non-metal  is  spoken 
of  later. 

Now  as  to  the  meaning  of  the  term  atomic  weight. 
We  have  spoken  quite  particularly  of  the  minute  particles, 
or  molecules,  of  which  the  physicist  believes  that  a  body 
is  made  up,  and  whose  relations  to  each  other  determine 
whether  the  body  is  a  solid,  a  liquid,  or  a  gas.  We  have 
also  seen  that  the  regular  form  of  a  crystal  is  due  to  the 
arrangement  of  these  molecules  as  they  are  marshaled  into 
place  by  the  attractive  forces  acting  between  them  when 
the  solid  is  formed.  Now  these  minute  molecules,  as  the 
chemist  believes,  are  made  up  of  simpler  particles,  often 
of  several  different  kinds  of  elements,  which  he  calls 
atoms.  It  is  the  relative  waight  of  the  atom  of  each 
substance  compared  with  that  of  the  lightest  substance 
known,  hydrogen,  that  is  called  its  atomic  weight.  This 


THE    CHEMICAL   CHARACTERS   OF   MINERALS.  103 

does  not  mean  that  the  chemist  can  actually  weigh  the 
atoms  themselves  that  form  the  minute  molecules  out  of 
which  a  body  is  built  up,  but  he  can  compare  the  weights 
of  two  masses,  for  instance  of  oxygen  and  hydrogen,  under 
such  conditions  that  he  is  sure  that  he  is  comparing  the 
same  number  of  atoms,  and  hence  he  obtains  the  relative 
masses  of  the  atoms;  or  he  may  obtain  the  same  result  in 
one  of  several  indirect  ways. 

It  is  found  invariably  true  that  when  the  different  ele- 
ments unite  to  form  a  certain  compound,  there  is  always  a 
definite  relation  between  the  amounts  by  weight  of  each 
element  which  enters,  and  that  these  weights  are  either 
the  atomic  weights  or  simple  multiples  of  them,  as  given 
by  the  number  of  atoms  present. 

Just  what  this  means  will  be  shown  by  some  examples. 
It  was  stated  above  that  the  chemist  could  decompose 
common  salt  into  sodium  and  chlorine.  Now  in  doing 
this  it  is  possible  to  find  how  much  by  weight  of  each  is 
present,  for  instance,  in  100  parts.  The  result  is  this : 

Sodium 39.32 

Chlorine  . .  .   60.68 


100.00 

But  the  numbers  39.32  and  60.68  are  in  the  ratio  of  23 
to  35.5  (39.32  :  60.68  =  23  :  35.5),  which  have  been  inde- 
pendently found  to  be  the  atomic  weights  of  these  two 
elements,  and  hence  it  is  evident  that  there  is  one  part,  or 
one  atom,  of  each  present  in  this  compound.  The  brief 
expression  for  the  composition  of  sodium  chloride,  or  the 


104  MINERALS,  AND   HOW   TO   STUDY   THEM. 

formula  as  it  is  called,  is  NaCl,  for  Na  is  the  symbol  for 
sodium  (natrium)  and  Cl  of  chlorine. 

The  formula  of  a  compound,  therefore,  gives  simply  the 
kinds  of  elements  present,  represented  by  their  initial  let- 
ters, that  is,  by  their  symbols,  with  small  numbers,  written 
usually  below,  to  show  how  many  parts  of  each,  that  is, 
how  many  atoms,  are  present. 

Again,  calcium  unites  with  chlorine  also,  and  the  com- 
pound, calcium  chloride,  analyzed  by  the  chemist,  gives: 

Calcium '.   36.04 

Chlorine 63.96 

100.00 

Here  the  numbers  36.04  :  63.96,  expressing  the  ratio  by 
weight  of  the  two  substances,  are  in  the  ratio  of  40  :  71  or 
40  :  2  X  35.5;  hence  the  compound  contains  one  atom  of 
calcium  and  two  of  chlorine,  and  the  brief  expression,  or 
formula,  for  it  is  Cad,. 

Two  other  examples  are  gold  chloride  and  tin  chloride, 
analyzed  with  the  following  results : 

Gold 64.87  Tin 45.26 

Chlorine..         .   35.13  Chlorine..       .  54.74 


100.00  100.00 

For  the  gold  chloride  the  ratio  of  64.87  :  35.13  is  as 
196.7:106.5  or  196.7:3x35.5;  hence  the  formula  is 
written  AuCl3.  Similarly  for  tin  chloride  the  ratio  of 
45.26  :  54  74  is  as  117.4  :  142  or  117.4  to  4  X  35.5;  hence 
the  formula  is  SnCl4. 


THE   CHEMICAL   CHARACTERS   OF   MINERALS.          105 

These  examples  illustrate  the  fact  that  the  atomic 
weights  of  the  given  elements  multiplied  by  the  number 
of  atoms  gives  the  amount  of  each  element  present  in 
the  given  compound.  They  also  show  another  important 
point:  it  is  seen  that  one  atom  of  chlorine  unites  with  one 
atom  of  sodium,  but  two  atoms  of  chlorine  with  one  of 
calcium,  three  with  one  of  gold,  and  four  with  one  of 
tin. 

Again,  in  hydrochloric  acid  the  formula  can  be  shown 
to  be  HC1;  in  other  words,  one  atom  of  hydrogen  is  pres- 
ent and  one  of  chlorine.  Water,  however,  has  the  formula 
HaO,  or  contains  in  a  molecule  two  atoms  of  hydrogen  and 
one  of  oxygen;  sulphureted  hydrogen  is  similarly  H2S. 
In  other  words,  one  atom  of  chlorine  is  here,  as  always, 
equivalent  to  one  of  hydrogen,  but  one  of  oxygen  or  one 
of  sulphur  is  equivalent  to  two  of  hydrogen. 

Further,  it  is  evident,  from  what  has  been  stated,  that 
the  formula  of  galena,  or  lead  sulphide,  must  be  PbS,  since 
one  atom  of  lead  unites  with  two  atoms  of  chlorine,  and  one 
of  sulphur  is  equivalent  to  two  atoms  of  hydrogen  or  chlor- 
ine; therefore  one  atom  of  lead  (Pb)  is  equivalent  in 
combining  power  to  one  atom  of  sulphur  (S). 

The  same  general  principle  can  be  extended  to  all  the 
other  elements;  or  in  other  words,  it  can  be  shown  how 
many  atoms  of  each  element  are  equivalent  in  forming 
compounds  to  one  of  hydrogen.  Thus  the  equivalence, 
as  it  is  called,  of  sodium  is  one,  of  calcium  two,  of  gold 
three  (also  sometimes  one),  of  tin  four.  In  the  compound 
SbCl6  the  equivalence  of  the  antimony  (Sb)  is  five,  but  in 
SbaO,  only  three. 


106  MINERALS,  AND   HOW   TO    STUDY   THEM. 

Some  complication  comes  in  from  the  fact  that  it  is  found 
that  the  same  substance  has,  within  certain  narrow  limits, 
different  equivalence  in  different  compounds,  as  noted 
above  of  gold  and  antimony;  thus,  too,  the  chemist  knows 
one  compound  FeO,  another  Fe.,0, ,  and  a  third  Fe02.  A 
good  deal  more  attention  must  be  given  to  the  matter  be- 
fore it  can  be  thoroughly  understood,  and  for  this  the 
student  must  have  a  good  course  in  chemistry,  including 
not  only  the  study  of  some  standard  book,  but  also  practical 
work  with  a  good  teacher  in  the  laboratory.  But  the  expla- 
nations given  should  suffice  to  make  it  pretty  clear,  first,  as 
to  what  the  elements  are ;  second,  what  is  meant  by  their 
atomic  weights;  and  third,  the  significance  of  their  com- 
bining power,  or  equivalence.  Some  further  explanations 
are  needed  as  to  chemical  compounds. 

The  distinction  between  a  chemical  compound  and  a 
simple  mixture  of  two  elements  is  well  illustrated  by  the 
air  we  breathe.  The  chemist  finds  by  analysis  that  it  is 
nearly  constant  in  composition,  containing  essentially  in  one 
hundred  parts  76.8  by  weight  of  nitrogen*  and  23.2  of  oxy- 
gen. A  little  water  vapor  is  also  present,  still  less  carbon 
dioxide. 

Is  the  air  a  chemical  compound  ?  The  answer  is  given 
at  once  that  it  is  not,  for  the  simple  reason  (and  there  are 
others  equally  conclusive)  that  the  ratio  of  76.8  to  23.2  is 
not  that  of  the  atomic  weights  of  the  two  elements  present, 
namely  14  :  16,  nor  of  any  simple  multiples  of  these. 

*  In  this  approximately  one  per  cent  has  been  shown  to  be  the 
new  element  argon,  which  in  many  characters  is  closely  related  to 
nitrogen. 


THE   CHEMICAL   CHAEACTERS   OF   MINERALS.          107 

There  are  indeed  several  compounds  of  nitrogen  and  oxy- 
gen know  to  the  chemist,  namely, 

N,0,        N.O.,        Na05; 

but  if  they  are  analyzed,  the  relative  amounts  by  weight  of 
nitrogen  and  oxygen  are  found  to  be  in  the  ratio  of 

2  X  14  :  16       2  X  14  :  3  X  16       2  X  14  :  5  X  16 
or  28  :  16  28  :  48  28  :  80 

One  further  point  must  be  mentioned  in  regard  to  the 
compounds  taken  for  illustration: 

Sodium  chloride,  NaCl. 
Calcium  chloride,  CaCla. 
Gold  chloride,  AuCls . 
Tin  chloride,  SnCl4. 

The  first  element  in  these  and  similar  formulas  is  a 
metal  and  the  second  a  non-metal;  the  first  is  said  to  be  the 
positive  element,  the  second  is  the  negative  element. 
Why  the  terms  positive  and  negative  are  introduced  is 
known  to  the  student  of  electricity,  for  he  has  learned  that 
in  the  decomposition  of  a  compound  by  the  electrical  cur- 
rent— a  very  powerful  means,  often  accomplishing  the  result 
when  other  methods  fail— one  element  always  goes  to  the 
positive  pole  or  electrode,  the  other  to  the  negative;  the 
former  is  hence  called  the  negative  element,  its  atoms  be- 
ing attracted  by  the  oppositely  electrified  positive  electrode, 
and  the  second  the  positive,  since  its  atoms  are  attracted  by 
the  negative  electrode.  Corresponding  to  this  the  metals, 
are  positive  in  nearly  all  their  compounds,  while  the  non- 
metals  are  negative,  and  further  the  semi-metals  are  some- 


108  MINERALS,    AND   HOW  TO   STUDY   THEM. 

times  positive,  sometimes  negative.  Remember  that  the 
positive  element  is  always  written  first;  this  will  be  clear 
from  the  examples  given  above,  and  in  the  following: 

FeO,        FeS,        PbO,        PbS,  etc. 

Again,  in  As.,0.,  and  AsaS3  arsenic  is  positive,  but  in  FeAs, 
and  CoAs3  arsenic  is  negative.  Similarly  among  the  metals 
there  are  some  which  in  compounds  with  certain  elements 
(as  oxygen,  sulphur)  are  always  positive,  while  when  com- 
bined with  certain  other  metals  they  may  be  negative. 

The  names  given  to  the  different  chemical  compounds 
are  in  most  cases  easy  to  learn  and  understand.  In  the  de- 
scription of  minerals,  in  the  pages  that  follow,  both  the 
chemical  names  and  the  formulas  are  given  so  as  to  famil- 
iarize the  student  with  each  method. 

When  there  are  two  or  more  compounds  of  the  same  ele- 
ments, the  name  is  usually  such  as  to  distinguish  between 
them.  Thus  PbO  and  PbOs  may  each  be  called  oxide  of 
lead  or  lead  oxide,  but  the  first  is  properly  lead  monoxide  * 
or  lead  protoxide,  and  the  second  lead  dioxide. 

The  following  are  other  examples: 

FeO,  iron  protoxide,  or  ferrous  f  oxide. 
Fea03,  iron  sesquioxide,  J  or  ferric  oxide. 

*Monoxide  means  an  oxide  containing  one  atom  (from  //oVoS.  single) 
of  oxygen;  dioxide  one  containing  two  atoms  (from  Si?,  twice);  protox- 
ide means  the  first  oxide  (TTpoSro?,  first)  because  the  first  or  lowest  of 
the  oxides  of  the  given  metal  in  amount  of  oxygen  present;  the  high- 
est oxide  is  sometimes  called  peroxide. 

f  The  terminations  -ous  and  -ic  are  frequently  used  for  the  lower 
and  higher  oxides  respectively. 

\  Sesquioxide  means  a  one-and-half  oxide,  because  the  ratio  of  oxy- 
gen to  metal  is  1$  :  1  or  3  :  2. 


THE   CHEMICAL   CHARACTERS   OF   MINERALS.  109 

FeS,,  iron  disulphide. 
SbaS3,  antimony  trisulphide. 
SnCl4,  tin  tetrachloride. 

The  following  are  a  few  special  names  with  which  it  is 
desirable  to  be  familiar: 

NaaO,  soda,  instead  of  sodium  oxide. 

K30,  potash,  instead  of  potassium  oxide. 

CaO,  lime,  instead  of  calcium  oxide. 

MgO,  magnesia,  instead  of  magnesium  oxide. 

BaO,  baryta,  instead  of  barium  oxide. 

A1Q03,  alumina,  instead  of  aluminium  trioxide. 

SiO,,  silica,  instead  of  silicon  dioxide. 

A  few  others  might  be  added  to  this  list. 

It  will  be  helpful  to  note  briefly  what  are  the  common 
kinds  of  compounds  found  among  minerals,  so  that  the 
statement  of  the  chemical  composition  and  formula,  given 
under  the  description  of  each  species,  may  have  a  definite 
meaning.  There  are  three  fundamental  divisions: 

I.  NATIVE  ELEMENTS. — This  is  the  simplest  case  of  all, 
that  of  the  elements,  a  few  of  which  occur  in  nature  and 
are  hence  called  native  elements,  as  native  gold,  native  sul- 
phur, etc. 

II.  SIMPLE  COMPOUNDS,  usually  of  two  elements;  there 
are  four  prominent  classes. 

(1)  Sulphides,  compounds  of  a  metal  with  sulphur,  as: 
Galena,  lead  sulphide,  PbS. 
Sphalerite,  zinc  sulphide,  ZnS. 
Pyrite,  iron  disulphide,  FeS,. 
Stibnite,  antimony  tmulphide,  Sb,S,. 


110  MINERALS,    AND   HOW  TO   STUDY   THEM. 

Similar  to  the  sulphides  and  closely  related  to  them  are 
the  rare  tellurides,  arsenides,  antimonides,  etc.,  as: 
Altaite,*  lead  telluride,  PbTe. , 
Niccolite,  nickel  arsenide,  NiAs. 
Breithauptite,  nickel  antimonide,  NiSb. 

(2)  Chlorides,  compounds  with  chlorine,  as: 

Halite  (rock  salt),  sodium  chloride,  NaCl. 
Cerargyrite,  silver  chloride,  AgCl. 

Similarly  the  rare  bromides  and  iodides  are  compounds 
with  bromine  and  with  iodine,  as: 

Bromyrite,  silver  bromide,  AgBr. 
lodyrite,  silver  iodide,  Agl. 

(3)  Fluorides,  compounds  with  fluorine,  as: 

Fluorite,  calcium  fluoride,  CaF9. 

(4)  Oxides,  compounds  with  oxygen,  as: 

Cuprite,  cuprous  oxide,  CuaO. 

Zincite,  zinc  oxide,  ZnO. 

Hematite,  iron  sesquioxide  (or  ferric  oxide),  Fe.,0,. 

Cassiterite,  tin  dioxide,  SnOa. 

The  examples  given  under  some  of  these  heads,  as  the 
sulphides  and  oxides,  illustrate  the  important  point  already 
spoken  of:  that  there  may  be  more  than  one  kind  of  com- 
pound, varying  in  the  number  of  atoms  present,  for  example 
of  oxygen,  as  Cu.,0,  ZnO,  Fe303,  SnOs.  Even  with  the  same 
metal  two  or  more  compounds  are  often  known,  though 
not  always  occurring  in  nature.  Thus,  besides  cuprite, 

*Some  rare  minerals,  not  elsewhere  metitioned,  are  for  the  sake 
of  completeness  included  in  this  list. 


THE   CHEMICAL   CHARACTERS   OF   MINERALS.          Ill 

CuaQ,  there  is  also  a  mineral  called  tenorite  whose  com- 
position is  CuO;  also  the  chemist  knows  FeO,  iron  protox- 
ide (or  ferrous  oxide),  while  Fe,03 ,  iron  sesquioxide  (or 
ferric  oxide),  is  the  common  mineral  hematite.  Further, 
there  is  magnetite,  whose  percentage  composition  is  ex- 
pressed by  the  formula  Fes04,  also  written  FeO.FeaOs;  in 
this  last  case  the  chemist's  view  of  the  composition  is  a  lit- 
tle too  complex  to  be  explained  here.  This,  however,  only 
illustrates  again  the  limitation  to  which  the  beginner  is 
subject,  since  he  cannot  expect  to  master  all  the  relations 
of  a  large  and  difficult  subject  without  much  hard  study. 
A  little  knowledge,  however,  is  useful  if  it  does  not  make 
the  one  who  possesses  it  imagine  that  he  has  a  deeper  un- 
derstanding of  nature's  laws  than  he  really  possesses. 

There  may  be  also,  under  each  of  these  heads,  com- 
pounds containing  more  than  one  metal,  or,  on  the  other 
hand,  more  than  one  negative  element,  as  arsenopyrite, 
FeAsS,  which  is  equivalent  to  FeAsa.FeSa.  Another 
simple  example  is  cryolite,  Na3AlFs,  which  is  equivalent 
to  3NaF.AlF9. 

We  may  provisionally  include  here  a  series  of  rather  rare 
compounds  among  minerals,  of  which  class,  though  nu- 
merous, only  a  very  few  are  alluded  to  in  this  book.  The 
best  example  is  tetrahedrite,  whose  formula  is  CuoSbaS7 , 
which  we  may  write  as  if  made  up  of  two  sulphides,  thus: 
4CuaS.SbaS,.  Pyrargyrite,  Ag,SbS,  or  3AgaS.SbaS3,  is  an- 
other example.  Strictly,  these  compounds  are  regarded  by 
the  chemist  as  similar  to  those  of  the  class  now  to  be  men- 
tioned called  Salts,  but  containing  sulphur  in  place  of 
oxygen. 


112  MINERALS,   AND  HOW  TO  STUDY   THEM. 

III.  COMPLEX  COMPOUNDS,  called  SALTS. — These  com- 
plex compounds  may  be  referred  back  in  each  case  to  some 
acid,  of  which  the  given  compound  is  said  to  be  a  salt.  In 
these  also  the  metal,  usually  written  first,  is  the  positive 
part,  the  remainder  the  negative,  in  the  electro-chemical 
sense  in  which  the  terms  were  used  on  page  107.  The 
metal  is  thought  of  as  taking  the  place  of  the  hydrogen 
atom  or  atoms  in  the  formula  of  the  given  acid.  The 
principal  classes  are: 

Carbonates,  salts  of  carbonic  acid,  H,C03 ,  in  which  some 
metal,  as  calcium,  lead,  etc.,  takes  the  place  of  the  two  hy- 
drogen atoms,  as : 

Calcite,  calcium  carbonate,  CaC03. 
Dolomite,  CaMg(COs)a  or  CaC03.MgCO,. 
Cerussite,  lead  carbonate,  PbC03. 

Sulphates,  salts  of  sulphuric  acid,  H2S04 ,  and  similarly 
chromates,  tungstates,  molybdates,  as: 

Barite,  barium  sulphate,  BaS04. 
Anglesite,  lead  sulphate,  PbS04. 
Crocoite,  lead  chromate,  PbCr04. 
Scheelite,  calcium  tungstate,  CaW04. 
Wulfenite,  lead  molybdate,  PbMo04. 

Phosphates,  mostly  salts  of  the  phosphoric  acid  H3P04. 
Closely  related  are  the  arsenates  and  vanadates;  less  closely 
the  rarer  antimonates  and  nitrates.  The  mineral  triphylite 
(not  common)  has  the  composition  LiFeP04  or  Li3P04. 
Fe,(P04),;  the  rare  mineral  xenotime  has  the  composition 
yttrium  phosphate,  YP04;  pucherite,  also  very  rare,  is  bis- 


THE   CHEMICAL   CHARACTERS   OF   MINERALS.  113 

muth  vanadate,  BiV04.  Other  examples  are  afforded  by 
the  common  minerals:  apatite,  essentially  calcium  phos- 
phate, Ca3P4Og  orCas(P04)s;  pyromorphite,  lead  phosphate, 
Pb3P208;  mimetite,  lead  arsenate,  Pb,As,,Og;  vanadinite, 
lead  vanadate,  Pb3  Va08.  In  all  of  these,  however,  the  com- 
position is  in  fact  a  little  more  complex  than  these  simple 
formulas  would  suggest,  since  chlorine  (and,  with  apatite, 
fluorine)  enters  in  small  amounts;  this  is  again  a  point 
which  it  will  not  be  attempted  to  explain  here. 

Less  common  than  the  preceding  classes  are  the  Tanta- 
lates  and  Niobates  (columbates),  as: 

Tantalite,   iron  tantalate,  FeTaa06. 
Columbite,  iron  niobate,  FeCbsOs. 

There  is  also  a  group  of  Borates,  mostly  very  rare  min- 
erals. 

A  very  large  and  important  but  complex  group  is  that 
of  the  Silicates.  These  are  salts  of  several  kinds  of  silicic 
acids,  the  commonest  of  which  are  metasilicic  acid,  HaSi03, 
and  orthosilicic  acid,  H4Si04,  as: 

Rhodonite,  manganese  metasilicate,*  MuSiO,. 
Willemite,  zinc  orthosilicate,  ZnaSi04. 

Other  silicates  are: 

Pyroxene  (diopside),  CaMg(Si03)a  or  CaMgSia08. 
Beryl,  Be,Ala(SiO,)6  or  BesAl2Sig018. 

These  are  metasilicates,  while  the  two  following  are  ortho- 
silicates  : 

*  The  designations  meta-  and  orlJio-  are  often  omitted,  and  the  com- 
pounds described  simply  as  silicates. 


114  MINERALS,   AND   HOW  TO  STUDY  THEM. 

Garnet  (grossularite),  Ca,Al8(Si04)s  or  Ca.Al.Si.O,, 
Zircon,  ZrSi04. 

Also 

Orthoclase,  KAlSi.O.. 
Albite,  NaAlSi308. 

And  again 

Andalusite,  AlaSi06. 

Besides  the  two  types  first  mentioned,  there  are  others, 
as  those  represented  by  orthoclase  (or  albite)  and  by  anda- 
lusite,  and  further  still  others,  many  of  them  very  com- 
plex and  in  some  cases  not  clearly  understood,  notwith- 
standing all  the  study  that  has  been  given  to  the  subject. 
All  of  these  points  require  much  more  knowledge  than  can 
be  demanded  of  a  beginner  in  mineralogy. 

As  illustrated  by  the  examples  given,  a  silicate  com- 
monly contains  more  than  one  metal,  perhaps  four  or  five 
or  even  more.  The  same  thing  is  also  true  of  the  com- 
pounds belonging  under  the  other  classes  of  salts,  though 
they  are  not  usually  so  complex  as  the  silicates;  with  them 
also,  in  some  cases,  two  of  the  negative  elements  may  be 
present. 

In  this  connection  it  is  also  important  to  understand 
that  the  relative  amounts  of  the  metals  present  may  in  a 
given  case  vary  widely,  as  the  amounts  of  calcium,  iron, 
etc.,  in  the  different  kinds  of  garnet.  For  example,  besides 
pure  calcium  carbonate,  CaCOs  (calcite),  and  magnesium 
carbonate,  MgCO,  (magnesite),  there  are  many  interme- 
diate compounds,  to  which  the  general  name  dolomite  is 
given,  containing  more  or  less  of  the  two  metals,  and  for 


THE  CHEMICAL  CHARACTERS  OF  MINERALS.          115 

them  the  formula  is  then  written  (Ca,Mg)COs.  For  nor- 
mal dolomite,  however,  in  which  one  atom  each  of  calcium 
and  magnesium  is  present,  the  formula  is  written  CaMgCaO, 
or  CaMg(C08)a;  the  difference  between  these  forms  (with 
and  without  a  comma)  should  be  noted.  So,  too,  the  three 
compounds  of  lead  mentioned  under  the  head  of  Phos- 
phates, namely,  the  minerals  pyromorphite,  mimetite, 
vanadinite,  have  many  intermediate  compounds  contain- 
ing in  varying  amount  two  of  the  negative  elements  phos- 
phorus, arsenic,  vanadium. 

The  formulas  of  these  salts  are  often  written  as  if  sepa- 
rated into  the  corresponding  simple  oxygen  compounds  or 
oxides,  as  CaO.COQ  instead  of  CaC03.  The  chemist  does 
not,  however,  believe  that  the  calcite  molecule  is  made  up 
of  these  two  parts,  only  this  method  of  writing  is  conven^ 
lent  because  these  are  the  parts  readily  obtained  by  analy- 
sis; in  this  particular  case  by  simply  heating  the  substance 
it  gives  off  CO, ,  and  CaO  is  left  behind. 

So,  too,  we  may  write  either 

CaMg(Si03)3  or  CaMgSi206  or  CaO.Mg0.2SiOa; 
CasAl,(Si04)3  or  Ca.Al.Si.O,,  or  3CaO.AlaOs.3SiOa; 
KAlSi308  or  KaO.Al209.6SiOa; 
Al,Si06  or  Als03.SiOa;  and  so  on. 

Hydrous  Compounds. — Finally,  there  are  also  a  large 
number  of  minerals  which  yield  water  when  heated;  some- 
times this  means  only  that  the  elements  of  water  (hydro- 
gen and  oxygen)  are  present  in  the  complex  compound 
and  they  combine  at  the  high  temperature.  This  is  true,  for 
example,  in  talc. 


116  MINERALS,    AND   HOW  TO  STUDY  THEM. 

In  the  majority  of  cases,  however,  the  water  is  believed 
to  be  present  as  water  of  crystallization,  as  if  water  mole- 
cules were  present  with  the  other  molecules  when  the 
compound  was  crystallized  out  from  the  solution.  These 
species  are  called  hydrous  compounds,  and  they  as  a  rule 
give  off  water  at  a  comparatively  low  temperature.  Thus 
gypsum  is  hydrous  calcium  sulphate,  and  its  composition  is 
expressed  by  the  formula 

CaS04  +  2HaO. 

Other  examples  of  hydrous  minerals  are  the  zeolites, 
among  silicates,  mentioned  near  the  end  of  the  chapter  on 
the  Description  of  Species.  Still  another  important  class 
are  the  hydrates,  or  hydrated  oxides — that  is,  oxides  which 
yield  water  when  heated.  A  good  example  is  brucite,  whose 
formula  is  written  Mg(OH)2  or  MgO.H20. 

Percentage  Composition. — It  was  shown  on  p.  104  that, 
from  the  proportion  by  weight  of  the  different  elements  ob- 
tained by  the  analysis  of  the  chemist,  the  chemical  formula 
could  be  deduced  by  the  aid  of  the  table  of  atomic  weights. 
Conversely,  if  the  formula  is  given,  the  percentage  composi- 
tion, or  the  amount  by  weight  of  each  element  or  group 
of  elements  present  in  the  compound  in  one  hundred  parts, 
can  be  easily  calculated.  Thus  sodium  chloride,  or  common 
salt,  has  the  formula  NaCl,  as  has  been  shown,  and  hence, 
taking  the  atomic  weights  of  sodium  and  chlorine,  the  re- 
lation of  the  weight  of  sodium  to  that  of  chlorine  is  as  ths 
numbers  23  :  35.5.  Now  adding  together  23  and  35.5  we 
obtain  56.5,  which  is  called  the  weight  of  the  molecule,  or 
molecular  weight.  Further,  by  the  rule  of  three,  if  58.5 


THE    CHEMICAL   CHARACTERS   OF   MINERALS.  117 

parts  of  the  compound  contain  23  parts  of  sodium,  100 
will  contain  39.32  parts: 


58.5  :  23     =  100  :  39.32,  or  —  =  39.32: 

00*0 


OK    K      V     1  00 

58.5  :  35.5  =  100  :  60.68,  or  — ^V^  =  60.68. 

o8.5 

The  percentage  composition  of  sodium  chloride  is,  there- 
fore: 

Na     39.32 

Cl      60.68 
100.00 

Again,  the  formula  of  stibnite,  SbaS3 ,  means  that  two 
atoms  of  antimony  (Sb)  unite  with  three  of  sulphur  (S). 
But  the  atomic  weights  of  antimony  and  sulphur  are  120 
and  32  respectively.  The  molecular  weight  is,  therefore, 

equal  to 

2  X  120  +  3  X  32  =  240  +  96  =  336. 

Hence  in  336  parts,  240  are  antimony  and  96  sulphur,  and 
to  find  the  amount  of  each  in  one  hundred  parts  we  have 

the  proportions 

94.0  v  100 

336  :  240  =  100  :  71.43,  or    *    =  71.43. 
ooo 

336  :  96  =  100  :  28.57,  or  96  *  10°  =  28.57. 

336 

The  percentage  composition  is,  therefore: 
Sb     71.43 
S      28.57 
100.00 

Again,  the  formula  of  one  kind  of  garnet  is  Ca3AlaSi30lt 
or,  as  it  may  be  written,  3CaO.AlaO,.3SiOa.  Taking  the 


118  MINERALS,    AND  HOW  TO  STUDY   THEM. 

second  form  and  finding  the  atomic  weights  for  each  ele- 
ment from  the  table,  adding  them  together  for  each  group 
of  atoms  and  multiplying  by  the  factor  given,  we  have: 

3CaO     =  3(40  +  16)  =  3  X  56  =  168 

Al,0,    =  2  X  27  +  3  X  16  =  54  +  48  =  102 
3SiO,    =  3(28  +  2  X  16)  =  3  X  60       =180 

450 
Hence  again,  by  the  rule  of  proportions: 

450  :  168  =  100  :  37.33 
450  :  102  =  100  :  22.67 
450  :  180  =  100  :  40.00 

The  percentage  composition  is,  therefore : 

Lime,  CaO  37. 33 

Alumina,  A1203    22.67 

Silica,  SiOa          40.00 

100.00 

If  desired  it  would  have  been  as  easy  to  deduce  the 
amounts  of  the  elements  Ca,  Al,  Si,  0  present,  but,  as  stated 
on  p.  115,  it  is  more  convenient  to  use  the  oxides  instead. 

CLASSIFICATION. 

There  are  various  methods  of  classification  that  may  be 
adopted  for  minerals.  The  strictly  scientific  way  is  to 
arrange  similar  compounds  together,  that  is,  first,  the  native 
elements;  then  the  sulphides,  the  oxides,  the  carbonates, 
and  so  on.  These  are  further  classified  by  the  relationships 
which  a  study  of  the  elements  and  of  the  crystalline  forms  of 
their  compounds  makes  known. 


THE   CHEMICAL   CHARACTERS   OF  MINERALS.  119 

For  example,  the  following  minerals  being  all  carbonates 
are,  on  a  strictly  scientific  method,  placed  in  the  same 
general  division: 

Calcite,  calcium  carbonate,  CaC03. 
Dolomite,  calcium-magnesium  carbonate,  CaMg(COs)». 
Magnesite,  magnesium  carbonate,  MgC03. 
Siderite,  iron  carbonate,  FeC03. 
Khodochrosite,  manganese  carbonate,  MnCO,. 
Smithsonite,  zinc  carbonate,  ZnC03. 

Further,  they  are  all  placed  side  by  side  in  the  same  group, 
called  the  Calcite  group,  because  they  have  the  same  general 
crystalline  form  and  very  nearly  the  same  angles,  e.g.,  all 
show  rhombohedral  cleavage  with  the  angle  varying  from 
%Q5°  to  107°.  This  is  called,  therefore,  an  isomorphous 
group,  having  like  form  *  and  analogous  composition. 

Another  series  of  minerals,  also  in  the  same  division  of 
carbonates,  form  a  second  isomorphous  group,  the  Aragou- 
ite  group: 

Aragonite,  calcium  carbonate,  CaCOs. 
Witherite,  barium  carbonate,  BaC03. 
Strontianite,  strontium  carbonate,  SrCOs. 
Cerussite,  lead  carbonate,  PbCOs. 
A  third  case  is  the  Barite  group  of  sulphates: 
Barite,  barium  sulphate,  BaS04. 
Celestite,  strontium  sulphate,  SrS04. 
Anglesite,  lead  sulphate,  PbS04. 
Also,  a  little  less  closely  related, 

Anhydrite,  calcium  sulphate,  CaS04. 

*  Isomorphous  is  from  i'tro?,  like,  and  tiop<f>jj,  form. 


120  MINERALS,    AND    HOW   TO    STUDY   THEM. 

The  Galena  group  (galena,  argentite,  etc.),  the  Apatite 
group  (p.  113),  the  Feldspar  group,  the  Mica  group,  are 
other  examples.  These  and  many  besides  are  described  in 
an  advanced  work  on  mineralogy. 

Another  method  of  classification  is  to  place  together  the 
different  compounds  of  each  metal,  as  all  the  compounds  of 
iron,  all  those  of  silver,  and  so  on.  Still  another  way  would 
be  to  put  the  metallic  ores  together,  the  gems,  and  so  on. 
Of  these  and  still  other  different  methods,  the  most  satis- 
factory for  us  is  the  second,  as  further  explained  on  a  later 
page  (p.  158  et  seq.). 

It  will  be  noticed,  in  the  cases  of  the  two  groups  taken  for 
illustration,  that  the  same  composition,  calcium  carbonate, 
CaCOs,  belongs  to  two  minerals,  calcite  and  aragonite. 
These  are  regarded  as  distinct  species  because  they  have  a 
different  crystalline  form  and  different  physical  characters, 
e.g.,  specific  gravity.  What  is  true  of  this  chemical  com- 
pound is  true  of  a  number  of  others.  Among  minerals,  such 
compounds  are  said  to  be  dimorphous  or  to  have  two  forma 


THE   USE   OF  THE   BLOWPIPE.  121 


CHAPTER  VI. 
THE  USE  OF  THE  BLOWPIPE. 

1.  GENERAL  DESCRIPTION  OF  APPARATUS, 

THE  chemist  in  the  laboratory,  as  has  been  already  ex- 
Dlained,  can  subject  a  mineral  specimen  to  a  process  of 
analysis,  and  in  this  way  discover,  first,  what  simple  sub- 
stances or  elements  it  contains,  and,  second,  in  what  pro- 
portion by  weight  they  are  present;  in  other  words,  he  can 
analyze  it.  This  has  been  done  many  times  in  the  case  of 
all  the  minerals  we  know,  and  the  result  has  been  to  show 
what  the  composition  of  each  species  is,  and  by  what  for- 
mula this  can  be  expressed. 

It  is  obvious  that  this  method  of  complete  analysis  is 
the  only  satisfactory  way  to  gain  a  complete  knowledge  of 
the  chemical  nature  of  a  given  mineral.  But  the  work  of 
the  chemist  is  slow  and  laborious,  and  it  is  often  important 
to  be  able  to  learn  something  about  the  composition  of  a 
mineral  more  quickly  and  by  an  easier  method.  This  can 
be  done  by  the  blowpipe,  supplemented  by  some  simple 
chemical  tests;  and  any  one  who  is  supplied  with  a  few 
tools,  and  who  has  the  patience  to  learn  to  use  them, 
can  accomplish  it.  The  results  of  this  blowpipe  analysis, 
taken  in  connection  with  the  study  of  the  physical  charac- 
ters of  a  given  specimen,  almost  always  suffice  to  enable  a 
mineralogist  who  has  a  fair  amount  of  knowledge  and  ex- 


122  MINERALS,  AND    HOW   TO   STUDY    THEM. 

perience  to  determine  what  it  really  is,  even  if  at  first  it 
was  entirely  unknown. 

The  following  list  includes  the  articles  that  are  most  es- 
sential for  this  work: 

1.  Lamp.  4.  Charcoal. 

2.  Blowpipe.  5.  Platinum  wire. 

3.  Platinum-pointed  forceps.      6.  Glass  tubes. 

Also  (7)  a  few  chemical  reagents  as  explained  beyond. 
After  some  words  of  explanation  about  each  of  these,  several 
other  appliances  which  it  is  also  convenient  to  have  will 
be  mentioned. 

1.  Lamp. — The  most  convenient  form  of  lamp  is  a  Bun- 
sen  gas-burner  (Fig.  149);  it  is  provided  with  a  special  jet 
(b  in  the  figure).     This  burner  can  be  connected  with  any 
ordinary  gas-jet  by  a  rubber  tube,  so  as  to  be  placed  on  the 
149  table  for  use.     In  the  Bunsen  burnei 

^        proper,  that  is,  when  the  jet  b  is  not 
inserted,  the  gas  mingles  in  the  tube 
with  the  air  which  enters  at  «,  and  they 
P      together  burn  at  the  top  in  a  very  hot 
flame,  but  one  which  gives  very  little 
light  and  which  deposits  no  soot  upon  a 
surface  of  cold  glass  or  porcelain.    This 
flame  is  used  by  the  chemist  in  the  lab- 
oratory, and  also  by  the  mineralogist  in 
heating  glass  tubes  as  described  beyond. 
Instead  of  the  Bunsen  burner  an  alcohol  lamp  may  be 
employed,  and  in  fact  was  long  used  by  the  early  chemists; 
alcohol,  however,  is  a  very  inflammable  substance,  so  that 


THE   USE   OF  THE   BLOWPIPE. 


123 


its  use  requires  much  care.  One  precaution  also  must  be 
observed  with  the  Bunsen  burner:  it  is  best  not  to  turn 
the  flame  down  low  (unless  the  end  of  the  tube  is  covered 
with  a  cap  of  wire  gauze),  for  if  this  is  done  the  flame  is 
liable  to  "  snap  down,"  that  is,  the  gas  may  ignite  within 
the  tube  just  above  a  (Fig.  149).  It  then  burns  with  a 
feeble  yellowish  flame,  yielding  a  disa-  150. 

greeable  odor,  and  the  tube  becomes  im- 
mediately very  hot.  This  is  dangerous, 
not  only  because  a  severe  burn  may 
result  from  touching  the  tube,  but,  still 
more,  because  if  left  a  few  moments  the 
rubber  tube  may  be  melted,  the  gas 
ignite  from  it,  and  a  serious  fire  be 
caused.  Hence  it  is  better  never  to  go 
out  of  the  room  and  leave  a  Bunsen 
flame  burning  even  for  a  few  minutes. 
In  a  laboratory  where  there  is  a  slate 
table  this  precaution  is  not  so  important. 

When  the  jet  b  is  inserted  in  the  tube 
of  the  Bunsen  burner  the  air-supply  a 
from  the  openings  below  is  cut  off  and 
the  gas  now  burns  at  the  top  with  the  usual  yellow  flame, 
here  flattened  by  the  shape  of  the  jet;  the  convenient 
flame  for  ordinary  use  is  about  one  and  a  half  inches  in 
height.  This  is  the  flame  to  be  used  with  the  blowpipe. 
Instead  of  this  gas-flame  a  good  stearine  candle  will  answer 
the  purpose  sufficiently  well,  or  an  oil  lamp  with  a  suit- 
able burner. 

2.  Blowpipe. — A  common  form  of  blowpipe  is  shown  in 


124  MINERALS,  AND   HOW  TO   STUDY   THEM. 

Fig.  150.  It  may  be  very  simple  and  inexpensive,  but 
should  have  an  air-chamber,  a,  to  collect  the  condensed 
moisture  from  the  breath.  A  separate  tip  (b),  either  of 
brass  or  platinum,  with  a  fine  hole,  is  often  used,  but 
it  is  not  absolutely  necessary.  The  essential  thing  is 
that  the  hole,  whether  in  the  tip  or  the  tube  itself,  should 
151.  be  large  enough  and  not  too  large,  and  also  that  it 
should  be  round  and  true,  so  that  a  moderate  pres- 
sure of  air  shall  suffice  to  blow  a  clear  blue  flame  (see 
Fig.  153).  A  trumpet-shaped  mouthpiece  (c)  is  usu- 
ally furnished,  but  some  prefer  to  dispense  with  it. 

3.  Forceps. — A  pair  of  steel  forceps  (Fig.  151)  is 
needed,  and  it  is  desirable  that  they  should  be  nickel- 
plated  to  prevent  rusting.  One  end  has  platinum 
points  at  d,  self-closing  by  a  spring,  so  that  the 
piece  of  mineral  to  be  heated,  placed  between  them, 
is  firmly  supported.  At  the  other  end  are  ordinary 
forceps  for  picking  up  small  fragments;  this  end 
should  never  be  inserted  in  the  flame.  A  caution  in 
regard  to  the  use  of  the  platinum  points  is  given  on  p.  130, 
for,  though  infusible,  they  can  be  easily  injured. 

4.  Charcoal. — Several  pieces  of  charcoal  are  needed. 
These  are  most  conveniently  rectangular  in  shape  (see 
Fig.  156)  and  about  four  inches  long,  an  inch  wide,  and 
three  fourths  of  an  inch  thick.  The  charcoal  must  burn 
without  snapping  and  must  leave  very  little  white  ash.  It 
is  so  difficult  to  obtain  really  good  charcoal  that  it  is 
well  worth  while  to  purchase  a  few  pieces  expressly  pre- 
pared for  the  purpose,  and  with  care  one  piece  will  last  for 


THE   USE   OF   THE   BLOWPIPE.  125 

many  experiments,  the  surface  being  rubbed  clean,  as  by  a 
file  or  knife,  after  each  use. 

5.  Platinum  Wire. — A  few  inches  of  platinum  wire,  of 
the  size  designated  No.  27,  usually  sold  for  this  purpose, 
are  needed;  directions  for  its  use  are  given  on  a  later  page. 
In  addition  to  the  wire,  a  small  piece  of  platinum  foil  is 
sometimes  useful. 

6.  Glass  Tubes.— Some  tubes  of  rather  hard  glass  are 
required;  it  is  convenient  to  have  two  sizes,  with  bores  of 
one  sixth  and  one  quarter  of  an  inch,  but  one  will  suffice. 
The  larger  size  can  be  cut  into  pieces  about  five  inches  in 
length;   the  tube  will  break  easily  if  a  single  scratch  is 
first  made  with  the  edge  of  a  three-cornered  file.     These 
tubes  are  to  be  used  as  open  tubes,  as  explained  later. 
Again,  pieces  a  little  longer,  say  six  inches,  and  of  the  size 
with  the  smaller  bore,  may  be  taken  and  held  with  the 
middle  point  in  the  hot  part  of  the  Bunsen  flame.     When 
the  glass  is  soft,  draw  the  two  ends  apart  by  a  quick  motion 
(without  twisting),  and  then  heat  each  long  tapering  end 
in  the  flame  and  pinch  it  off  short  while  hot,  using  for  this 
the  steel  end  of  the  forceps.     In  this  way  two  closed  tubes 
will  be  made  from  each  piece;    a  considerable   number 
should  be  made  and  kept  in  a  closed  box  for  use.     A  tube 
must  be  clean  inside  and  out,  and  should  not  be  used  twice. 

7.  Fluxes  and  other  Chemical  Reagents. — The  chemical 
reagents  needed  are  the  fluxes  *  borax  (sodium  tetraborate), 
soda  (sodium  carbonate),  and  salt  of  phosphorus,  or  micro- 
cosmic  salt  (phosphate  of  soda  and  ammonia).     Each  of 

*  So  called  because  they  help  in  the  melting  or  fusion  of  the  sub- 
stance under  examination. 


126  MINERALS,  AND   HOW  TO   STUDY   THEM. 

these  may  be  kept  in  a  round  wooden  pill-box,  or  in  a  small 
bottle  with  a  glass  stopper.  A  little  potassium  bisulphate, 
to  be  kept  in  a  glass  bottle,  is  occasionally  needed.  Small 
bottles  of  hydrochloric,  nitric,  and  sulphuric  acids  are  also 
useful,  and  one  of  a  solution  of  cobalt  nitrate;  these  bottles 
may  conveniently  have  a  glass  dropping-tube  with  a  bulb  in 
the  place  of  the  ordinary  glass  stopper. 

Test-paper  is  also  required,  cut  up  into  small  strips,  both 
turmeric-paper  and  blue  litmus-paper.  The  yellow  tur- 
meric-paper is  turned  broivn  by  an  alkali,  such  as  soda, 
while  the  Hue  litmus-paper  is  turned  red  by  an  acid  or 
acid  fumes,  as  of  sulphur  dioxide  in  the  open  tube.  Eed 
litmus-paper  turns  blue  with  an  alkali,  but  the  turmeric- 
paper  is  better. 

In  addition  to  the  above,  the  following  articles  will  be 
found  very  convenient,  though  not  all  of  them  quite  so  es- 
sential: 

A  small  hammer  having  a  square  face  with  sharp  edges; 
also  a  steel  anvil  an  inch  or  two  long,. 

A  horseshoe  magnet  (Fig.  152),  the  place  of 
which  may  be  taken  by  a  magnetized  knife- 
blade. 

A  small  agate  mortar  and  pestle;  also  a 
steel  diamond  mortar  (one  in  which  the  pestle 
fits  tightly)  in  which  a  hard  mineral  can  be 
pulverized  without  loss  of  the  fragments. 
A  pair  of  cutting  pliers. 
A  three-cornered  file. 
A  few  small  watch-glasses  are  convenient;  also  several 
small  dishes  of  glass  or  porcelain  (smooth  butter-plates  are 


THE   USE   OF  THE   BLOWPIPE.  127 

very  good)  to  hold  the  fragments  of  the  mineral  under  ex- 
amination; several  test-tubes;  a  porcelain  dish,  or  casser- 
ole, in  which  a  substance  can  be  heated  with  acid.  Also, 
if  chemical  tests  proper  are  to  be  tried,  a  wash-bottle  (for 
distilled  water),  a  bottle  of  ammonia,  and  some  filter-paper. 

Before  beginning  to  experiment  it  is  best  to  put  a  thick 
sheet  of  cardboard,  covered  each  time  with  a  fresh  piece  of 
white  paper,  upon  the  table  and  place  the  lamp  upon  this. 
A  slate  or  a  sheet  of  plate  glass  is  even  better  than  the 
cardboard. 

The  student  must  remember  also  that  the  acids  men- 
tioned are  powerfully  corrosive  in  their  action,  staining 
and  finally  destroying  any  fabric,  as  clothes  or  the  carpet, 
which  they  are  allowed  to  touch.*  Moreover,  the  fumes 
from  the  acids  when  hot  are  injurious;  for  any  extended 
series  of  strictly  chemical  trials  it  is  almost  essential,  there- 
fore, to  have  some  of  the  conveniences  of  a  laboratory. 
Still  another  caution  is  needed:  do  not  put  away  a  piece 
of  charcoal  after  use  until  it  is  quite  certain  that  no  fire 
lingers  in  it. 

2.  How  TO  USE  THE  BLOWPIPE. 

The  first  thing  in  the  use  of  the  blowpipe  is  to  learn  to 
blow  a  hot,  steady  flame.  Place  the  tip  of  the  blowpipe  close 
to  or  just  within  the  flame  as  shown  in  Fig.  153,  directing 
it  slightly  downward,  and  blow  through  the  tube.  The 
blast  of  air  will  direct  the  flame  into  a  thin  cone,  and  with 

*  In  case  of  accident  the  effect  of  the  acid  can  often  be  neutralized 
by  the  prompt  application  of  ammonia  or  carbonate  of  soda,  which 
may  afterward  be  washed  out  with  a  little  water. 


128  MINERALS,  AND    HOW   TO   STUDY   THEM. 

a  little  practice  a  clear  blue  flame  quite  free  from  yellow 
will  be  the  result.  This  flame  is  much  hotter  than  the  or- 
dinary gas-flame,  and  when  the  blowpipe  is  in  skillful 
hands  it  is  hot  enough  to  melt  a  fine  platinum  wire.  The 
hottest  part  is  just  at  the  extremity  of  the  blue  flame 
(shaded  in  Fig.  153). 

It  seems  difficult  at  first  to  blow  a  continuous  steady 
flame,  but  it  is  really  very  easy.  It  is  only  necessary  to 
continue  slowly  to  breathe  through  the  nose  while  the 
pressure  of  the  cheeks  upon  the  reservoir  of  air  kept  all 
the  time  in  the  mouth  prolongs  the  blast.  This  pressure 
need  not  be  great — not  enough  to  tire  the  cheek-muscles 
sensibly  except  after  a  long  time;  if  fatigue  soon  comes,  it 
is  because  the  student  is  unskillful  or  has  a  bad  blowpipe. 

It  is  not  wise,  however,  to  give  too  much  thought  to  the 
learning  of  the  art  of  steady  blowing;  this  will  come 
quickly  with  practice.  At  the  same  time  it  will  not  do 
to  be  careless  about  the  character  of  the  flame;  the  stu- 
dent is  ready  to  go  on  when  he  can  take  a  thin  sliver  of 
orthoclase  and  without  great  difficulty  melt  the  edges. 

An  important  distinction  must  be  made  between  the 
reducing  flame  and  the  oxidizing  flame.  The  flame  in 
general  consists  of  two  parts:  the  inner  blue  cone,  and  the 
outer  almost  invisible  envelope  extending  far  beyond.  In 
the  former  the  gas  is  only  partly  burned;  there  is  a  de- 
ficiency of  oxygen,  and  a  substance  which  at  that  tem- 
perature can  part  with  its  oxygen  is  reduced.  Here  the 
reducing  effect  is  to  rob  of  oxygen,  as  when  oxide  of  nickel, 
NiO,  is  changed  to  metallic  nickel  (Ni);  or  iron  sesqui- 
oxide  (FeaOs)  is  changed  to  iron  protoxide  (FeO). 


THE    USE   OF   THE    BLOWPIPE. 


129 


In  the  outer  part  of  the  flame,  on  the  other  hand,  there 
If  an  excess  of  oxygen  from  the  surrounding  air,  and  the 
tendency  is  to  give  oxygen,  or  to  oxidize.  Here  the  lower 
oxide  of  manganese,  MnO,  is  changed  to  the  higher  oxide., 
Mn,08. 

This  distinction  between  the  action  of  the  two  parts  of 
the  flame  is  very  important  in  a  certain  class  of  experi- 
ments. The  student  must  notice  further  that  to  blow  a 
good  strong  oxidizing  flame  the  tip  of  the  blowpipe  should 
be  placed  just  inside  the  gas-flame,  as  indicated  in  Fig. 
153;  the  flame  is  then  free  from  any  yellow,  and  the  sub- 
153.  154. 


stance  under  experiment  is  to  be  held  well  beyond  the  end 
of  the  blue  cone,  at  d. 

For  a  good  reducing  flame,  on  the  other  hand,  the  tip 
should  be  a  little  outside  of  the  gas-flame  (Fig.  154),  so 
that  a  little  yellow  follows  the  flame  down,  above  the  blue 
cone;  the  substance  is  held  at  d,  within  the  blue  cone,  and 
best  more  or  less  surrounded  by  the  yellow  flame.  The 
experiment  described  on  p.  138  with  manganese  will  show 
the  learner  with  what  success  he  is  following  the  direc- 
tions here  given. 

In  the  following  pages  the  different  methods  of  exami- 
nation with  the  aid  of  the  blowpipe  are  described  fully. 
The  student  should  take  them  up  in  order,  going  through 


130  MINERALS,  AND   HOW   TO   STUDY    THEM. 

as  many  as  possible  of  the  trials  with  the  minerals  sug- 
gested and  endeavoring  to  obtain  the  results  described  as 
closely  as  he  can.  It  is  essential  that  the  material  used 
for  the  experiments  should  be  pure. 

3.  EXAMINATION  IN  THE  FORCEPS. 

A  small  fragment  of  a  mineral,  held  in  the  platinum 
points  of  the  forceps,  may  be  tested  to  see  whether  it  can 
be  melted,  and,  if  so,  whether  easily  or  with  difficulty.  At 
the  same  time  it  may  be  observed  that  the  mineral  imparts 
a,  color  to  the  flame  which  will  give  information  as  to  its 
composition,  while  other  phenomena,  as  detailed  below, 
may  also  be  noted. 

And  here  a  few  important  suggestions  must  be  made. 
It  is  very  necessary  to  remember  that  while  platinum  can 
not  be  injured  by  the  heat  of  the  blowpipe  flame,  nor 
attacked  by  the  ordinary  acids  used  by  the  chemist,  it  may 
yet  be  easily  injured.  A  mineral  containing  antimony  or 
arsenic,  if  fused  in  the  forceps,  may  destroy  the  platinum 
points,  for  these  metals  form  a  very  fusible  alloy  with 
platinum.  Hence  it  is  desirable  to  try  minerals  about 
which  there  is  question — especially  a  mineral  with  metallic 
luster — in  the  closed  tube  or  on  charcoal  first,  and  if  there 
are  fumes  given  off,  caution  is  needed. 

In  any  case  it  is  a  good  rule  never  to  let  the  fused  part 
of  the  mineral  fragment  come  in  contact  with  the  plati- 
num; for  it  may  adhere  to  the  points  in  an  inconvenient 
way,  even  if  not  capable  of  doing  any  permanent  harm, 
and  thus  much  time  be  wasted  in  cleaning  them. 


THE   USE   OF  THE   BLOWPIPE.  131 

Take  now  a  little  sliver,  if  possible  with  a  fhin  edge,  of  a 
piece  of  barite  or  heavy  spar;  place  it  between  the  platinum 
points,  letting  the  edge  project  well  beyond  them;  blow 
a  clean  blue  flame  with  the  blowpipe,  and  just  in  front  of 
this  (in  the  oxidizing  flame,  see  Fig.  153)  insert  the  min- 
eral. It  will  be  seen  to  melt  rather  easily  to  a  white 
opaque  glass;  at  the  same  time  the  flame  beyond  will  be 
streaked  with  a  pale  yellowish  green,  which  is  character- 
istic of  the  element  barium.  Further,  if  the  fused  end, 
after  it  has  cooled,  be  placed  upon  a  piece  of  moistened 
turmeric-paper,  it  will  be  seen  to  turn  it  brown,  showing 
the  presence  of  an  alkaline  earth. 

If  a  piece  of  a  barite  crystal  is  taken,  it  is  very  likely  to 
break  violently  into  fragments  when  the  flame  is  thrown 
upon  it.  This  is  called  decrepitation  and  is  not  uncom- 
mon, especially  with  crystallized  minerals.  It  can  often 
be  prevented  by  heating  the  fragment  quite  slowly  at  first, 
but  in  some  cases  it  is  necessary  to  begin  by  reducing  the 
mineral  to  a  fine  powder,  then  mix  it  with  a  drop  of  water 
in  the  agate  mortar,  and  finally  support  the  thick  paste  so 
formed  on  a  loop  at  the  end  of  the  platinum  wire. 

Scale  of  Fusibility. — The  method  of  experiment  de- 
scribed gives  in  the  first  place  an  approximate  determina- 
tion of  the  melting-point  or  degree  of  fusibility.  The 
following  scale  is  used  to  define  the  fusibility  of  the  differ- 
ent minerals: 

1.  Stibnite  (must  be  heated  on  charcoal) :  fusible  in  the 
ordinary  gas-flame  even  in  large  fragments. 

2.  Natrolite :  fusible  in  fine  needles  in  the  ordinary  gas- 
flame,  or  in  larger  fragments  in  the  blowpipe-flame. 


132  MINERALS,  AND  HOW  TO  STUDY  THEM. 

3.  Almandite,  or  iron-alumina  garnet:  fusible  to  a  glob- 
ule without  difficulty  with  the  blowpipe,  if  in  quite  thiu 
splinters. 

4.  Actinolite :  fusible  to  a  globule  in  thin  splinters. 

5.  Ortltoclase:  thin  edges  can  be  rounded  without  great 
difficulty. 

6.  Bronzite :  fusible  with  difficulty  on  the  finest  edges. 
The  following  list  gives  the  names  of  some  minerals, 

most  of  them  common,  with  the  degree  of  fusibility  of  each 
according  to  this  scale.  It  is  repeated  here  that  for  miner- 
als with  metallic  luster  the  trial  should  be  in  charcoal. 

Stibnite,  galena 1. 

Cryolite,  apophyllite,  pyromorphite 1.5 

Amblygonite,  witherite,  prehnite,  arsenopyrite 2. 

Ehodonite,  analcite 2.5 

Gypsum,  barite,  celestite,  fluorite,  epidote 3. 

Oligoclase 3.5 

Albite 4. 

Apatite,  hematite,  magnetite 5. 

Bronzite 6. 

Infusible :  quartz,  calcite,  topaz,  sphalerite,  graphite. 

It  may  be  interesting  here  to  add  the  temperatures  (in 
degrees  Centigrade)  at  which  the  prominent  metals  fuse, 
that  is,  pass  from  the  solid  to  the  liquid  state: 

Mercury —39°  Antimony...     450° 

Silver 1020° 

Tin 230°  Copper 1090° 

Bismuth 320°  Gold 1100° 

Lead 330°  Iron 1500° 

Zinc-.  .  420°  Platinum  .    .   2000° 


THE    USE   OF   THE   BLOWPIPE.  133 

The  student  must  be  warned  that  the  method  of  express- 
ing the  fusibility  of  a  mineral,  by  referring  it  to  the  scale 
given,  is  not  exact.  The  results  obtained  in  different  cases 
will  depend  upon  the  size  and  shape  of  the  fragment 
taken,  the  conductivity  for  heat,  also  obviously  upon  the 
skill  of  the  experimenter. 

Flame  coloration. — Besides  the  fusibility,  this  experi- 
ment with  a  fragment  of  barite  in  the  forceps  serves  to 
prove  the  presence  of  barium  by  the  color  given  to  the 
flame.  It  is  found  that  a  considerable  number  of  sub- 
stances are  characterized  in  the  same  way,  hence  the  flame 
coloration  becomes  a  simple  and  important  means  of  quali- 
tative blowpipe  chemical  analysis. 

Color  of  the  Flame. — The  following  is  a  list  of  the  colors 
likely  to  be  observed  and  the  substances  to  which  they  are 

due: 

(  Carmine-red... . Lithium. 

RED  ...  \  Purple-red Strontium. 

I  Yellowish  red    .Calcium. 
YELLOW Sodium. 

(  Yellowish  green.  Barium. 

I  Siskine-green . . .  Boron. 
GREEN.  <j  EmeraM_green_Gop^eTf 

[  Bluish  green. .  ..Phosphoric  acid  and  phosphates. 
Greenish  blue . .  .Antimony. 

Whitish  blue. .  ..Arsenic. 
BLUE..  4    .  , .    ., 

Azure-blue Copper  chloride. 

Violet Potassium. 

It  may  be  noted  here  that  the  blue  flame  of  copper 
chloride  is  sometimes  used  as  a  test  for  chlorine.  For  ex- 


134  MINERALS,  AND    HOW   TO   STUDY   THEM. 

ample,  if  powdered  pyromorphite  be  mixed  with  a  little 
cuprite,  also  in  the  form  of  powder,  and  the  mixture  be 
fused  together  upon  charcoal,  a  blue  flame  will  be  obtained 
for  a  moment,  indicating  that  the  pyromorphite  contains 
some  chlorine. 

A  bluish-green  flame  is  also  given  by  tellurium,-  yellow- 
green  by  molybdenum;  whitish  green  by  metallic  zinc. 
Blue  flames  are  also  given  by  lead  (on  charcoal)  and  by 
selenium. 

The  color  characteristic  of  a  given  substance  is  often 
masked  by  another;  thus  the  green  due  to  the  boron  in 
torax  is  concealed  by  the  stronger  yellow  of  the  soda,  but 
may  be  seen  clearly  if  a  drop  of  sulphuric  acid  is  placed  on 
the  substance  before  heating.  Similarly,  the  same  treat- 
ment shows  the  bluish  green  of  the  phosphorus  in  salt  of 
phosphorus,  which  also  contains  soda.  Further,  a  difficultly 
fusible,  or  infusible,  mineral  is  often  not  sufficiently  de- 
composed by  simple  heat  to  show  the  flame-color,  and 
hence  a  more  complex  method  is  called  for;  thus  a 
fragment  of  apatite,  first  moistened  by  sulphuric  acid,  gives 
in  the  forceps  the  green  for  phosphoric  acid.  Again,  ortho- 
clase  may  be  mixed  with  an  equal  bulk  of  powdered  gypsum, 
a  paste  made  with  a  little  water,  and  this  fused  on  a  clean 
platinum  wire,  when  the  violet  flame  of  potassium  becomes 
visible,  unless  indeed  masked  by  soda,  in  which  case  a 
piece  of  blue  glass  will  extinguish  the  yellow  and  allow  the 
violet  to  be  seen.  Further,  the  test  for  boron  applicable 
to  the  silicate  tourmaline  is  given  under  the  description  of 
that  species. 

In  the  case  of  carbonates,  as  calcite,  strontianite,  etc.,  a 


THE   USE   OF  THE   BLOWPIPE.  135 

drop  of  hydrochloric  acid  will  result  in  the  formation  of  a 
little  chloride  (of  calcium,  strontium,  etc.),  which  colors 
the  flame  more  intensely.  It  is  obvious  that  great  care  is 
needed  to  keep  the  platinum  points  of  the  forceps,  or  the 
wire,  perfectly  clean.  A  wire  which  has  been  handled  or 
moistened  with  saliva  will  always  give  a  yellow  (sodium) 
flame. 

At  the  same  time  that  he  tests  the  fusibility  and  flame 
color  of  a  mineral,  the  student  must  keep  his  eyes  open  to. 
note  other  attendant  phenomena.  Some  of  the  points  he 
may  observe  are  the  following: 

The  fragment,  instead  of  fusing  quietly,  may 

(a)  Swell  up,  throw  out  little  globules  or  curling  pro- 
cesses, as  stilbite. 

(b)  Intumesce ;  that  is,  bubble  up  and  then  fuse,  as 
scapolite  and  most  zeolites. 

(c)  Exfoliate ;  that  is,  swell  up  and  open  out  in  leaves, 
as  apophyllite  and,  even  more,  vermiculite. 

(d)  Glow  brightly,  without  melting,  as  calcite. 

Also  the  fragment  after  being  heated  must  be  examined 
to  see  whether,  if  fused,  the  glass  is  clear,  full  of  bubbles 
(then  often  called  Uebby,  or  vesicular),  or  even  black; 
whether  it  has  changed  color,  even  if  not  fused ;  whether 
it  is  magnetic  (due  to  iron) ;  whether,  if  placed  on  moist- 
ened turmeric-paper,  it  turns  it  brown,  as  is  true  of  an 
alkali,  as  soda,  or  an  alkaline  earth,  as  lime  or  baryta. 

Finally,  a  few  infusible  substances  turn  blue  when,  after 
being  heated,  they  are  moistened  with  a  drop  of  cobalt 
solution  and  again  heated;  this  is  true  of  cyanite,  kaolin, 
and  other  infusible  minerals  containing  alumina.  Calamine 


136  MINERALS,  AND   HOW  TO    STUDY   THEM. 

(zinc  silicate)  also  turns  blue  under  these  circumstances. 
A  blue  obtained  with  fusible  minerals  treated  in  this  way 
may  be  simply  due  to  the  cobalt  (see  p.  138).  Further, 
some  minerals  containing  magnesia,  as  the  hydrate  brucite, 
with  similar  treatment  turn  a  pale  pink. 


4.  USE  OF  THE  PLATINUM  WIRE. 

Wind  the  platinum  wire  about  a  small  piece  of  card,* 
leaving  some  three  inches  free;  then  bend  the  end  par- 
155.   tially  around  so  as  to  make  an  open  loop  nearly  as 
O    big  as  a  very  small  pea  (Fig.  155),  it  is  now  ready 
for    use.     Heat    this    in    the    blowpipe-flame    and 
dip  it  into  the  borax;  some  will  adhere,  which  is 
then  to  be  fused  together  to  a  colorless  glass.     Then 
add  a  little  more,  and  repeat  the  operation  until  the 
bead  is  clear  and  round  and  fills  the  loop  entirely; 
it  must  not  be  too  big  or  it  will  fall  off  the  wire. 
Now  take  a  little  cuprite  or  malachite  in  very  fine  frag- 
ments, heat  the  bead  and  bring  it  in  contact  with  one  of 
them;   it  should  adhere,  and  when  heated  again  in  the 
flame— it  should  now  be  held  in  the  oxidizing  flame— it 
will  slowly  dissolve  and  disappear  (note  the  emerald-green 
flame  of  the  copper).     Now  examine  the  bead,  and  if  the 
quantity  of  the  copper  mineral  was  very  small  it  will  be 
found  to  be  clear  green  while  hot,  and  blue  when  cold. 
If  a  larger  amount  was  taken  or  is  now  added,  and  the 

*  Instead  of  this,  the  wire  may  perhaps  better  be  cut  into  short 
pieces  (say  three  inches  or  less  in  length),  and  each  fused  into  a  glass 
tube  drawn  out  as  described  on  p.  125. 


THE   USE   OF  THE   BLOWPIPE.  137 

bead  again  heated,  it  may  remain  green  on  cooling,  or  even 
be  so  deeply  colored  as  to  appear  black  and  opaque.  In  the 
last  case  the  color  may  often  be  seen  if  the  bead  is  heated 
hot  and  quickly  flattened  out  by  the  pressure  of  the  agate 
pestle.  In  every  case  it  is  best  to  commence  with  a  very 
minute  portion  and  then  add  more,  rather  than  to  take  so 
much  that  the  bead  is  black.  If  too  much  has  been  taken, 
it  is  best  to  shake  off  the  bead  from  the  wire  by  a  sudden 
motion  when  hot,  or  break  it  when  cold,  and  commence 
again. 

Suppose  now  that  the  bead  is  deep  green  and  contains  a 
relatively  large  amount  of  copper,  or  is  saturated,  as  it  is 
called.  Hold  it  in  the  reducing  flame  (see  Fig.  154),  and 
heat  again.  Now  the  oxygen  needed  to  burn  the  gas 
will  be  taken  in  part  from  the  oxide  of  copper  (CuO)  in  the 
bead  and  part  of  it  will  be  changed  or  reduced  to  the 
lower  oxide  of  copper,  Cu,0,  which  will  show  red  and 
opaque  when  the  bead  is  cold. 

After  having  obtained  this  red  opaque  bead  it  will  be 
good  practice  to  heat  it  again  carefully  in  the  oxidizing 
flame  until  the  bead  is  once  more  perfectly  clear  and 
green  in  color.  The  color  of  the  borax  bead  thus  is  a  test 
which  can  prove  the  presence  of  copper,  and  similarly  also 
of  a  number  of  the  other  metals. 

The  following  is  a  list  of  the  common  metallic  oxides 
and  the  colored  beads  that  they  yield.  The  distinction 
between  the  oxidizing  flame  (0.  F.),  which  is  to  be  used 
first,  and  the  reducing  flame  (R.  F.)  is  to  be  carefully  ob- 
served. Thus  nickel  gives  a  violet  bead  in  the  0.  F.  from 
the  oxide  NiO,  while  in  the  R.  F.  this  becomes  gray  and 


138 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


muddy  from  metallic  nickel.  Also  manganese  gives  in  the 
0.  F.  a  deep  wine-red,  from  the  presence  of  the  sesquioxide 
(Mna03),  and  in  the  R.  F.  it  becomes  colorless,  the  pro- 
toxide (MnO)  being  formed. 


Colors  of  the  Borax  Beads. 

(1)  OXIDIZING  FLAME   (o.  F.). 

Bed,   red-brown,     Chromium    (Cr2Oi)    when    hot;    yellowish  green 
and  brown.  when  cold. 

Manganese  (MnaO3)  amethystine- red;  violet  when 

hot. 
Iron  (FeaOs)  when  hot;  yellow  when  cold,  when 

saturated. 

Nickel  (NiO)  red-brown  to  brown  when  cold;  vio- 
let when  hot. 

Uranium  (UO3)  when  hot;  yellow  on  cooling. 
Green.  Copper  (CuO)   when   hot;    blue  when    cold,   or 

bluish  greeu  if  highly  saturated. 

Chromium  (Cr2Oa)  when  cold;  yellow  to  red,  hot. 

Yellow.  Iron  (Fe2O3)  when  hot;  pale  yellow  or  colorless 

when  cold  (if  saturated,  red-brown  and  yellow). 

Uranium  when  hot  and  feebly  saturated;  paler  on 

cooling. 
Chromium  (CraO»)  when  hot  and  'eebly  saturated; 

yellowish  green  when  cold. 
Blue.  Cobalt  (CoO)  hot  and  cold. 

Copper  (CuO)  when  cold;    green  when  hot.     If 

highly  saturated,  bluish  green  when  cold. 
Violet.  Nickel  (NiO)  hot;  red-brown  to  brown  cold. 

Manganese  (Mn2O3)  hot;  violet-red  cold. 

(2)  REDUCING  FLAME  (B.  F.). 

Colorless.  Manganese  (MnO),  or  with  a  faint  rose-color. 

Bed.  Copper  (Cu3O)  opaque. 

Green.  Iron  (FeO)  bottle-green. 

Chromium  (Cr2O3)  emerald-green. 

Uranium  (U3O8);  yellowish  green  if  saturated 
Blue.  Cobalt  (CoO). 

Gray  or  turbid.       Nickel  (Ni). 


THE    USE   OF   THE    BLOWPIPE.  139 

It  may  be  added  that  many  of  the  common  metals,  both 
0.  F.  and  K.  F.,  as  silver,  zinc,  lead,  etc.,  further  silica, 
etc.,  give  colorless  beads. 

To  expel  the  arsenic,  antimony,  or  sulphur,  the  process 
of  roasting  is  performed.  This  consists  in  heating  the 
powdered  substance  on  charcoal  cautiously,  so  as  not  to 
fuse  it,  first  in  the  0.  F.,  then  in  the  E.  F.,  and  repeating 
this  a  number  of  times  patiently.  The  mineral  should 
still  be  in  the  state  of  a  powder  at  the  end. 

It  must  be  noticed  that  when  two  metals  are  present 
together,  the  color  of  one  may  not  be  seen;  thus,  if  iron, 
nickel,  and  cobalt  are  all  in  the  substance  under  examina- 
tion, the  colors  are  observed  in  the  order  given,  if  each 
metal  in  turn  is  oxidized  off  by  skillful  treatment  on  char- 
coal with  successive  portions  of  borax. 

Salt  of  phosphorus  is  used  especially  for  some  of  the 
rarer  elements;  in  general  the  results  are  nearly  the  same 
as  with  borax.  The  following  may  be  noted : 

Titanium  (TiOJ,  in  0.  F.  yellow  hot,  colorless  cold ;  in 
R.  F.  yellow  hot,  fading  out  to  a  delicate  violet. 

Chromium  (Cr203),  in  0.  F.  red  when  hot,  dirty  green 
on  cooling,  fine  green  when  cold;  in  R.  F.  nearly  the  same. 

Uranium,  in  0.  F.  yellow  when  hot,  yellowish  green 
when  cold ;  in  R.  F.  yellowish  green  when  hot,  and  green 
cold. 

Vanadium,  in  0.  F.  dark  yellow  when  hot,  paler  on 
cooling;  in  R.  F.  brownish  red  when  hot,  chrome-green  on 
cooling. 

Molybdenum,  in  0.  F.  yellowish  green  hot,  paler  on 
cooling;  in  R.  F.  dirty  green  hot,  green  when  cold. 

Niobium    (columbium) ,  in    0.  F.   yellow  when   hot   if 


140  MINERALS,  AND   HOW  TO  STUDY  THEM. 

slightly  saturated,  colorless  on  cooling;  in  R.  F.  dirty  blue 
when  hot,  and  blue  cold  if  highly  saturated. 

It  is  also  to  be  noted  that  silica  (as  quartz)  or  a  silicate 
does  not  dissolve  entirely  in  salt  of  phosphorus,  as  in 
borax,  but  the  metallic  oxides  dissolve  out  and  leave  a 
skeleton  of  silica  floating  around  in  the  bead. 

The  making  of  the  salt-of -phosphorus  bead  on  the  wire 
calls  for  a  little  skill.  The  salt  contains  considerable 
water  of  crystallization,  and  when  first  heated  it  melts  in 
this  and  becomes  so  fluid  as  to  fall  from  the  wire;  hence 
small  quantities  should  be  taken  and  fused  till  the  boiling 
has  ceased  before  more  is  added;  also,  and  this  is  particu- 
larly important,  during  the  preliminary  process  hold  the 
bead  just  over  the  flame,  not  in  it,  then  the  vapors  ex- 
pelled will  support  the  fluid  bead  and  keep  it  from  falling. 

Soda  is  particularly  useful  on  charcoal  in  reducing  the 
metallic  compounds  as  described  below.  With  manganese 
in  the  oxidizing  flame  soda  gives  a  fine  bluish-green  but 
opaque  bead.  The  soda  beads  in  general  are  opaque,  but 
with  silica  a  clear  glass  may  be  obtained. 

5.  USE  OF  THE  CHARCOAL. 

A  piece  of  charcoal  such  as  has  been  described  on 
p.  124  is  very  useful  in  the  chemical  examination  of 
minerals  with  the  blowpipe.  It  forms  a  support,  in  the 
first  place,  upon  which  a  deposit  may  be  formed  of  the 
volatile  compound  formed  by  heating  the  substance. 
Besides  this  the  glowing  carbon  has  what  has  been  called 
(p.  128)  a  powerful  reducing  effect,  that  is,  it  takes 


THE   USE   OF  THE   BLOWPIPE.  141 

oxygen  (or  other  elements)  away,  and  sometimes  without 
other  means  this  suffices  to  produce  the  metal  from  its 
compound.  Thus  cuprite  (CuaO),  or  red  oxide  of  copper, 
is  reduced  to  metallic  copper  by  heating  on  charcoal; 
similarly  chalcocite  (Cu2S)  may  be  treated  with  the  same 
result;  cerargyrite  (AgCl)  yields  metallic  silver. 

Instead  of  charcoal  a  plate  of  plaster  of  Paris,  or  one  of 
the  metal  aluminium,  is  sometimes  used  as  a  support  in 
place  of  the  charcoal,  but  it  is  not  often  that  there  is  any 
advantage  in  this  substitution. 

The  way  in  which  the  charcoal  is  used  and  the  chemical 
principles  involved  will  be  made  clear  by  a  few  examples. 


The  fragment  of  mineral  used  should  be  placed  near 
one  end  of  the  rectangular  piece  of  charcoal  (see  p.  124 
and  Fig.  156)  and  held  so  that  the  flame  will  sweep  down 
the  full  length.  This  will  give  a  volatile  substance,  if  one 
is  formed  during  the  heating,  the  best  opportunity  to 
deposit  when  on  the  cooler  surface.  It  is  not  necessary  to 
make  a  deep  hole;  often  a  short  scratch  made  across  the 
charcoal  with  a  sharp  edge  is  sufficient:  against  this  the 
fragment  is  blown  by  the  flame.  When  the  fragment  per- 
sists in  jumping  off  it  may  sometimes  be  held  in  place  by 
a  very  little  borax  fused  to  it  beforehand  while  it  is  held 
in  the  forceps. 


142  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Take  a  piece  of  stibnite  (sulphide  of  antimony,  Sb.,8,), 
and  place  it  on  the  charcoal  and  heat  (gently  at  first,  for  it 
is  likely  to  fly  to  pieces,  or  decrepitate).  It  will  fuse  very 
easily,  for  it  stands  first  in  the  scale  of  fusibility,  and 
while  melting,  and  with  further  heating  after  fusion,  it  will 
give  off  a  cloud  of  white  fumes.  These  become  dense,  and 
collect  as  a  white  coating  over  the  coal;  the  black  surface 
seen  through  the  white  will  on  the  edges  give  the  effect  of 
blue.  If  the  heating  is  continued,  the  mineral  will  en- 
tirely disappear,  or,  in  other  words,  it  is  entirely  volatilized. 
Such  a  coating  is  called  a  sublimate,  and  in  this  case  it 
consists  of  the  antimony  trioxide  (Sb.,0,)  formed  by  the 
union  of  the  antimony  with  the  oxygen  of  the  air;  the 
compound  produced  at  the  same  time  \>y  the  combination 
of  the  sulphur  and  oxygen,  sulphur  dioxide  (SO,),  goes  off 
as  a  gas  in  the  air.  If  now  the  reducing  flame  is  thrown 
for  a  moment  against  the  white  coating,  it  is  burned  off 
with  a  bluish  flame.  The  action  of  the  flame  is  to  reduce 
the  oxide  to  the  metal  (Sb),  which  is  instantly  volatilized, 
and  as  it  goes  off  it  is  again  oxidized. 

Again,  take  a  fragment  of  orpiment,  sulphide  of  ar- 
senic (AsaS3),  and  treat  it  in  the  same  way.  The  result  is 
somewhat  similar;  it  fuses  easily,  giving  white  fumes  (of 
As203),  and  it  is  also  entirely  volatile.  But  now  a  strong 
disagreeable  odor  will  be  perceived  as  the  fumes  are 
formed;  this  is  usually  described  as  a  garlic,  or  alliaceous, 
odor;  it  is  characteristic  of  arsenic,  and  is  always  produced 
when  the  metal  is  volatilized  and  arsenic  trioxide  (As.,0,) 
formed.  The  odor  serves  to  distinguish  the  two  cases  just 
described;  but  more  than  this,  the  white  coating  will  be 


THE   USE   OF  THE   BLOWPIPE.  143 

perceived  to  lie  this  time  much  farther  from  the  flame 
than  the  oxide  of  antimony,  because  it  is  more  volatile 
and  can  be  deposited  only  where  the  coal  is  comparatively 
cool. 

A  third  trial  may  be  made  with  arsenopyrite.  It  gives 
off  as  it  is  heated  a  cloud  of  white  fumes  with  the  same 
peculiar  penetrating  garlic  odor,  and  the  white  coating  of 
arsenic  trioxide  forms  at  a  distance  on  the  coal.  There 
is,  however,  a  residue  in  this  case  which  soon  fuses  to  a 
grayish  black  globule  which  when  cold  is  found  to  be 
magnetic,  proving  the  presence  of  iron.  The  mineral  con- 
sists of  iron,  sulphur,  and  arsenic  (the  formula  is  FeAsS). 
Part  of  the  sulphur  is  driven  off  (as  S02),  and  after  some 
time  all  the  arsenic,  while  a  magnetic  compound  of  iron 
and  sulphur  (with  perhaps  a  little  residue  of  arsenic)  is 
left  behind. 

Another  trial  may  be  made  with  sphalerite  or  zinc 
blende,  but  to  succeed  now  the  mineral  should  be  pulver- 
ized first,  since  it  is  infusible  before  the  blowpipe  and 
the  compound  is  only  with  difficulty  decomposed  on  char- 
coal. A  little  of  the  powder  placed  in  the  scratch  and 
carefully  heated  (lest  it  be  blown  away)  will  cohere 
together  and  presently,  if  the  flame  is  hot,  a  coating  will 
be  formed  over  the  powder  and  just  about  it  on  the  coal. 
If  the  powdered  mineral  is  first  mixed  with  two  or  three 
times  its  volume  of  sodium  carbonate,  it  is  then  more 
easily  decomposed  and  the  sublimate  obtained.  This 
coating,  which  consists  of  the  oxide  of  zinc  (ZnO),  has  a 
bright  canary-yellow  color  when  hot,  but  becomes  white 
on  cooling.  If  the  coating  is  thin,  it  might  sometimes  be 


144  MINERALS,  AND    HOW   TO   STUDY   THEM. 

mistaken  for  the  charcoal  ash  (although  good  charcoal 
gives  very  little),  and  confirmatory  evidence  can  be  ob- 
tained by  letting  a  drop  of  cobalt  nitrate  fall  upon  it. 
when,  if  again  heated  in  the  oxidizing  flame,  it  will  assume 
a  bright  green  color  characteristic  of  zinc.  Still  again,  if 
the  coating  of  zinc  oxide,  as  at  first  obtained,  is  heated 
with  the  reducing  flame,  it  is  reduced,  the  zinc  volatilized 
yielding  a  characteristic  bluish-green  flame.  It  may  be 
repeated  here  that  a  fragment  of  calamine  (zinc  silicate) 
yields  a  similar  sublimate,  but  the  mineral  itself  becomes 
blue  when  heated  after  being  moistened  by  the  cobalt 
solution. 

A  fragment  of  galena  should  also  be  tried  on  charcoal. 
It  will  fuse  very  easily,  and  immediately  about  it  there  will 
form  a  yellow  coating  of  the  oxide  of  lead  (PbO),  while 
farther  off  there  will  be  a  white  coating  of  lead  sulphate 
(PbS04)  formed  by  the  union  of  the  PbO  and  S0a  in  the 
presence  of  the  oxygen  of  the  air. 

Further,  lead  is  what  is  called  an  easily-reducible  metal; 
that  is,  its  compounds  are  rather  easily  changed  to  the 
metallic  state  under  the  action  of  heat,  as  on  charcoal; 
hence  continued  heating  yields  globules  of  metallic  lead. 
A  little  soda  on  the  galena  hastens  the  production  of  the 
metal,  and  at  the  same  time  it  is  noted  that  the  yellow 
coating  is  more  distinct,  while  the  white  fumes  are  nearly 
absent,  for  now  the  soda  unites  with  the  sulphur  of  the 
galena. 

Again,  a  fragment  of  ruby  silver — either  the  dark  red 
(pyrargyrite)  containing  silver,  antimony,  and  sulphur,  or 
the  light  red  (proustite)  containing  silver,  arsenic,  and 


THE   USE    OF   THE    BLOWPIPE.  145 

sulphur — may  be  tried.  It  will  fuse  easily,  and  give  off 
white  fumes  of  either  the  oxide  of  antimony  or  arsenic 
according  as  to  which  mineral  was  in  hand,  and  the  dis- 
tinction is  easily  noted  as  before  described.  A  black 
globule  will  be  left  behind;  and  if  now  some  sodium  car- 
bonate be  poured  over  this  on  the  coal,  and  the  mass  heated 
persistently  till  the  globule  is  fused  in  it,  presently,  after 
rather  long-continued  blowing,  a  white  globule,  or  perhaps 
several,  will  be  seen  moving  about  in  the  fused  soda.  By 
removing  the  fused  mass  from  the  charcoal  and  crushing  it 
in  a  mortar,  the  metallic  silver  is  readily  separated.  It 
will  be  found  that  the  globule  is  white  like  silver,  and 
remains  bright  (not  oxidizing  readily),  and  it  is  malleable. 
That  it  really  is  silver  may  be  proved  by  chemical  means, 
for  it  dissolves  easily  in  nitric  acid,  and  the  addition  to  the 
solution  of  a  drop  of  hydrochloric  acid  causes  a  white 
curdy  precipitate  of  silver  chloride  to  separate  at  once. 
The  reddish  coating,  on  the  coal,  of  the  silver  oxide  formed 
during  the  process  is  sometimes  distinct.  From  some 
silver  minerals,  as  the  chloride,  cerargyrite  (also  called 
horn-silver),  the  metal  is  obtained  at  once  by  heating  on 
charcoal. 

A  mineral  containing  copper  will  yield  metallic  copper 
on  charcoal  when  heated  with  the  soda.  This  may  be 
either  in  small  globules  or  as  a  thin  crust.  When  exposed 
to  the  air  the  copper  becomes  coated  with  the  black  oxide, 
but  it  is  easily  recognized,  being  malleable  in  the  anvil, 
and  showing  when  rubbed  its  peculiar  red  color. 

Cassiterite  or  tin-stone,  first  powdered,  for  it  is  an  infusi- 
ble and  refractory  mineral,  and  then  heated  with  soda,  will 


146  MINERALS,  AND   HOW   TO   STUDY   THEM. 

give  minute  malleable  globules  of  metallic  tin.  These  are 
at  first  nearly  as  white  as  silver,  but  soon  oxidize  and 
become  dull;  with  a  little  nitric  acid  in  a  watch-glass  they 
yield  an  insoluble  white  powder  of  tin  dioxide.  The 
reduction  is  more  easily  accomplished  if  potassium  cyanide 
is  added  to  the  mixture,  but  it  is  a  very  poisonous  sub- 
stance, and  its  use  hardly  to  be  recommended  outside  of 
the  laboratory.  The  tin  globules  may  not  be  very  con- 
spicuous in  the  soda,  but  are  easily  separated  from  the  soda 
by  crushing  and  washing  in  a  mortar;  the  soda  and  char- 
coal are  washed  off  and  the  heavy  tin  particles  left  behind. 

Besides  the  coatings  mentioned  in  these  examples,  the 
following  must  be  mentioned : 

Bismuth  gives  a  volatile  sublimate,  which  is  dark  orange- 
yellow  when  hot  and  lemon-yellow  when  cold.  Mixed  with 
equal  parts  of  potassium  iodide  and  sulphur  and  heated  in 
the  0.  F.,  a  beautiful  red  sublimate  of  bismuth  iodide  is 
deposited. 

Molybdenum  gives  a  sublimate  which  is  yellow  when  hot 
and  white  on  cooling;  this  is  volatile  in  the  0.  F.,  leaving 
a  copper-red  stain  of  the  oxide;  if  touched  for  a  moment 
with  the  R.  F.  a  beautiful  azure-blue  is  obtained. 

Cadmium  gives  a  sublimate  red-brown  near  the  frag- 
ment and  orange-yellow  at  a  distance  from  it;  the  subli- 
mate is  volatile. 

Selenium  and  selenides  give  a  very  disagreeable  odor  (like 
decaying  horse-radish)  which  is  highly  characteristic. 

Soda  is  also  used  on  charcoal  with  the  group  of  com- 
pounds called  sulphates,  to  prove  the  presence  of  sulphur. 
A  little  of  the  pulverized  mineral  (as  barite)  fused  with 


THE   USE   OF   THE   BLOWPIPE.  147 

the  soda  yields  a  mass  of  a  liver-brown  color  (called  hepar), 
which,  removed  from  the  coal  and  placed  with  a  drop  of 
water  on  a  silver  coin,  will  stain  it  black.  This  is  explained 
by  the  action  upon  the  silver  of  the  sodium  sulphide 
formed  on  charcoal.  It  is  essential  that  the  soda  itself 
should  be  free  from  sulphur;  and  further,  since  this  may 
be  contained  in  the  illuminating  gas  employed,  a  prelimi- 
nary trial  should  be  made  with  soda  alone;  if  this  gives  no 
action  on  the  silver,  then  the  final  result  with  this,  if  show- 
ing the  presence  of  sulphur,  can  be  trusted. 


6.  USE  OF  THE  CLOSED  AND  OPEN"  TUBES. 

The  tubes  in  blowpipe  work  are  chiefly  used  in  the 
examination  of  minerals  which  yield  on  heating  a  volatile 
substance;  this  in  most  cases  is  condensed  in  the  colder 
part  of  the  tube.  There  is  an  important  distinction  to  be 
observed  between  the  use  of  the  closed  and  the  open  tube. 
The  closed  tube  contains  but  very  little  air,  and  this  is 
driven  out  with  the  first  puffs  of  gas  from  the  heated 
mineral,  and  hence  what  goes  on  takes  place  without  much 
effect  from  the  oxygen  of  the  air. 

In  the  case  of  the  open  tube,  on  the  contrary,  if  held  in 
the  proper  inclined  position,  there  is  a  constant  stream  of 
hot  air  (that  is,  of  oxygen)  which  passes  up  the  tube  and 
over  the  heated  mineral  fragment.  A  few  examples  will 
show  how  this  principle  is  applied. 

Place  a  little  fragment  of  sulphur  in  the  closed  tube  and 
heat  it  gently.  At  once  it  is  fused  and  converted  into 
sulphur  vapor  which  rises  in  the  tube  and  soon  condenses, 


148  MINERALS,  AND   HOW  TO   STUDY  THEM. 

giving  a  dark  orange-red  ring  of  liquid  sulphur,  which 
becomes  light  yellow  as  it  cools  and  solidifies.  Here  there 
has  been  no  change,  simply  the  volatilization  of  the 
sulphur. 

Now  place  a  fragment  in  a  rather  large  open  tube,  about 
an  inch  from  the  end;  incline  the  tube  as  much  as  possi- 
ble without  causing  the  fragment  to  slip  out,  and  heat  it 
very  slowly.  The  sulphur  fuses  as  before,  but  the  hot 
oxygen  which  passes  over  it  unites  with  it,  forming 
sulphur  dioxide  (SOJ,  an  invisible  gas  which  rises  through 
the  tube  and  comes  out  of  the  open  end,  giving  the  usual 
sulphur  odor  (it  should  be  'inhaled  with  a  little  caution); 
further,  the  acid  fumes  of  this  gas  will  turn  a  piece  of  blue 
litmus-paper  bright  red.  It  is  difficult  to  heat  the  sulphur 
slowly  enough  to  prevent  the  formation  also  of  a  ring,  as 
in  the  closed  tube,  simply  because  it  is  easily  volatile;  that 
is,  it  goes  off  into  gas  very  readily,  and  the  oxygen  can 
hardly  be  supplied  fast  enough  to  oxidize  it  all. 

As  a  second  example,  take  a  small  piece  of  as  pure  cinna- 
bar as  can  be  obtained  (it  often  occurs  with  clay  as  a  gangue, 
as  it  is  called,  and  this  may  give  off  water  and  obscure  the 
result).  The  cinnabar  is  sulphide  of  mercury,  HgS,  a 
substance  which  is  converted  into  vapor  when  heated  out 
of  contact  with  the  air.  In  the  closed  tube  we  get  at  once 
a  black  ring,  or  sublimate,  of  mercury  sulphide  which,  like 
the  sulphur,  was  first  volatilized  and  then  condensed  where 
the  tube  was  cooler.  This  black  coating  becomes  reddish 
if  rubbed.  In  addition  to  it  there  may  be  also  a  faint  gray 
deposit  above  of  metallic  mercury,  because  of  the  small 
amount  of  air  in  the  tube  at  the  start  (see  beyond). 


THE   USE   OF  THE   BLOWPIPE.  149 

Now  place  a  fragment  in  the  open  tube  and  heat  it,  this 
time  also  very  slowly  and  carefully.  Gradually  the  cinna- 
bar disappears,  while  the  sulphurous  fumes  can  be  per- 
ceived at  the  end  of  the  tube,  as  in  the  other  case.  But 
more  than  this,  a  little  above  the  fragment  a  faint  deposit 
begins  to  form,  growing  more  and  more  distinct,  and 
finally,  when  seen  by  reflected  light,  it  appears  as  a  shining 
mirror.  This  is  metallic  mercury  in  the  form  of  minute 
globules  coating  the  glass;  that  it  is  mercury  can  be  proved 
even  to  the  skeptical  by  cutting  the  tube  carefully  near  the 
deposit  (by  first  scratching  it  with  a  file)  and  then  rub- 
bing the  deposit  with  a  match-stick.  The  minute  globules 
unite  to  form  a  few  large  ones  which  will  run  out  of  the 
tube,  when  tipped  up,  and  on  to  the  hand.  As  before 
remarked,  a  little  of  the  metallic  mercury  may  be  noted 
for  the  same  reason  in  the  closed  tube. 

It  is  not  difficult  to  explain  what  has  happened  in  this 
case.  The  hot  oxygen  passing  over  the  heated  mineral  has 
united  with  the  sulphur  to  form  sulphur  dioxide  (SOJ, 
while  the  mercury  thus  left  alone  has  been  driven  off  as 
vapor  by  the  heat  and  collected  where  the  cube  was  cool 
enough  to  allow  of  its  condensation.  Very  likely  in  this 
case  too,  unless  the  heating  is  very  slow,  a  little  sulphide 
of  mercury  will  go  off  without  change  and  form  a  black 
ring  in  the  closed  tube,  but  by  gradually  heating  this, 
keeping  the  tube  in  the  same  position,  it  is  driven  up  the 
tube,  more  and  more  of  the  sulphur  being  oxidized,  until 
nothing  but  the  pure  metallic  mirror  of  the  mercury  is 
left. 

This  experiment  succeeds  best  if  the  tube  is  first  heated 


150  MINERALS,  AND    HOW   TO   STUDY   THEM. 

quite  hot  a  little  above  the  mineral  and  then  the  heating 
of  the  fragment  carried  on  very  slowly  and  carefully.  If 
the  powdered  cinnabar  be  mixed  with  soda  (first  dried  to 
expel  the  water),  and  then  introduced  into  the  dosed  tube* 
and  heated,  a  sublimate  of  metallic  mercury  is  very 
readily  obtained. 

Again,  take  a  fragment  of  galena;  in  the  closed  tube  it 
undergoes  no  change  and  no  sublimate  is  formed.  If, 
however,  another  fragment  is  placed  in  the  open  tube,  al- 
though no  sublimate  is  produced  here,  some  of  the  sulphur 
is  oxidized  and  the  sulphurous  fumes  can  be  perceived  by 
the  odor  or  by  their  reddening  effect  on  litmus-paper. 
This  method  is  consequently  a  general  method  of  testing 
for  sulphur  in  the  class  of  compounds  called  sulphides. 

A  fragment  of  orpiment,  sulphide  of  arsenic,  As2S,, 
heated  in  the  closed  tube  is  melted,  volatilized,  and  forms 
a  beautiful  red  ring  of  sulphide  of  arsenic.  Heated  in  the 
open  tube  (very  slowly),  both  sulphur  and  arsenic  are 
oxidized;  the  sulphur  gives  as  always  S0a,  while  the  ar- 
senic yields  a  white  deposit  of  minute  octahedral  crystals 
of  arsenic  trioxide  (As203)  spangling  in  the  light.  This 
sublimate  is  very  volatile  and  hence  may  be  driven  farther 
and  farther  up  the  tube  when  heated. 

Arsenopyrite,  FeAsS,  in  the  closed  tube  gives  a  trace  of 

*  In  a  case  like  this  where  the  substance  is  in  powder,  it  can  be 
introduced  into  the  tube  without  soiling  the  tube  (which  is  quite  an 
important  matter)  if  a  little  trough  be  made  by  folding  once  a 
narrow  strip  of  paper  ;  then  place  the  substance  in  this  and  insert  it 
in  the  tube  carefully,  this  being  held  in  a  horizontal  position  ; 
now  when  the  tube  is  turned  into  a  vertical  position  the  powder 
will  fall  to  the  bottom  and  the  paper  can  be  removed. 


THE   USE   OF  THE   BLOWPIPE.  151 

a  white  sublimate  of  the  oxide  of  arsenic,  but  more  dis- 
tinctly at  first  a  dark  red  deposit  of  sulphide  of  arsenic 
(AsaS3),  which,  if  the  heating  is  stopped  and  the  tube 
allowed  to  cool,  becomes  a  rich  red  color.  1 1  the  heating 
is  continued,  the  arsenic  now  goes  off  alone  and  forms  a 
shining  mirror  of  crystalline  scales  of  metallic  arsenic. 
The  residue  is  magnetic  and  consists  of  iron  and  sulphur 
chiefly. 

In  the  open  tube,  heated  slowly,  part  of  the  sulphur 
goes  off  as  sulphur  dioxide,  while  the  arsenic  gives  a  white 
crystalline  deposit  of  As20s,  and  the  same  magnetic  res- 
idue as  before  is  left  behind.  This  case  illustrates  again 
the  important  difference  between  the  use  of  the  open  and 
closed  tube. 

Another  good  example  is  given  by  stibnite,  SbaSs.  In 
the  closed  tube  it  is  all  volatilized  and  gives  a  dark  red 
sublimate,  most  of  which  is  a  complex  compound  called 
an  antimony  oxysulphide  (2Sb.,S3.Sb,03).  The  presence 
of  this  is  explained  by  the  fact  that  the  oxygen  in  the 
small  amount  of  the  air  contained  in  the  tube  is  enough  to 
unite  with  the  sulphide  of  antimony  and  form  the  com- 
pound named. 

In  the  open  tube  both  sulphur  and  antimony  are  oxi- 
dized. The  sulphur  gives  sulphur  dioxide  (SO,)  and  the 
antimony  gives  antimony  trioxide  (Sb.,03),  which  forms  as 
a  dense  white  powdery  deposit  which  is  not  volatile  when 
heated  by  the  flame.  It  is  thus  easily  distinguished  from 
the  arsenic  trioxide,  which  is  crystalline  and  spangles  in 
the  light  instead  of  being  a  dull  powder;  while  the  oxide 
of  arsenic  too  is,  as  before  stated,  very  volatile.  The  dis- 


152  MINERALS,  AND   HOW   TO    STUDY   THEM. 

tinction  between  the  sublimates  of  antimony  and  arsenic 
formed  on  charcoal  should  be  recalled  (see  pp.  142,  143). 

Of  other  rarer  substances  it  may  be  mentioned  that 
selenium  gives  a  dark  red,  nearly  black,  sublimate  in  the 
closed  tube  with  its  peculiar  disagreeable  odor  at  the  open 
end;  in  the  open  tube  the  sublimate  is  steel-gray,  the 
upper  edge  red  with  perhaps  white  volatile  crystals  of  the 
oxide. 

Tellurium  in  the  closed  tube  condenses  in  small  drops 
with  metallic  luster;  in  the  open  tube  a  gray  sublimate  is 
formed  which  fuses  to  colorless  drops,  becoming  solid  on 
cooling. 

Pyrite,  iron  disulphide,  FeS,,  when  heated  in  the  closed 
tube  gives  off  about  half  its  sulphur  which  condenses  in  a 
ring  like  that  just  described.  Heated  slowly  in  the 
open  tube,  the  sulphur  which  is  driven  off  may  be  all  oxi- 
dized to  S02.  A  magnetic  residue  is  left  in  both  cases. 

A  few  other  uses  of  the  tubes  must  be  mentioned  here. 

A  mineral  containing  water,  when  heated  in  the  closed 
tube,  gives  off  the  water  vapor  which  condenses  as  drops 
of  water  in  the  upper  part  of  the  tube.  A  change  in  the 
appearance  of  the  mineral  may  take  place  at  the  same 
time.  Thus  a  piece  of  limonite,  or  hydrated  oxide  of  iron, 
gives  off  its  water  and  turns  red,  for  it  is  now  the  an- 
hydrous oxide  of  iron,  like  hematite,  which  has  a  red 
powder.  In  a  few  cases,  as  with  some  sulphates,  the  water 
has  an  acid  reaction  and  turns  blue  litmus-paper  red.  It 
may  be  added  that  the  higher  oxide  of  manganese  (MnOa 
— the  mineral  pyrolusite)  gives  off  oxygen  in  the  closed 
tube. 


THE   USE   OF  THE   BLOWPIPE.  153 

Fluorine  is  usually  tested  for  in  the  closed  tube,  the 
powdered  mineral  being  mixed  with  previously  fused  bi- 
sulphate  of  potash  and  then  heated;  the  hydrofluoric 
acid  given  off  attacks  the  glass  or,  as  it  is  usually  ex- 
pressed, etches  it. 

A  few  minerals,  as  fluorite,  phosphoresce  in  the  tube, 
that  is,  give  out  a  yellow  or  green  light  when  held,  after 
slight  heating,  in  a  dark  spot.  Also,  as  another  phenom- 
enon sometimes  noted,  the  fragment  when  heated  in  the 
tube  may  glow  brightly. 

The  more  of  the  experiments  with  the  minerals  named 
the  student  performs  the  better,  for  knowledge  thus  ob- 
tained by  experience  is  much  better  than  knowledge 
learned  from  a  book.  A  list  of  the  minerals  particularly 
useful  for  blowpipe  work  is  given  in  an  Appendix  at 
the  close  of  this  book.  Any  one  who  has  mastered  the 
elements  of  blowpipe  work  and  who  is  interested  in  learn- 
ing more  should  turn  to  a  manual  of  blowpipe  analysis, 
where  he  will  find  many  more  tests  and  reactions  and 
more  minute  directions  for  the  work  in  general.  The  work 
of  Professor  Brush  on  Determinative  Mineralogy  may  be 
particularly  recommended. 


7.  CHEMICAL  EXAMINATION  BY  ACIDS  AND  OTHER 
REAGENTS. 

In  addition  to  the  various  methods  of  chemical  examina- 
tion .already  described  which  can  be  made  by  means  of  the 
blowpipe,  there  are  a  few  other  chemical  tests  so  easy  to 
apply  that  the  mineralogist  should  be  in  a  position  to  use 


154  MINERALS,  AND  HOW  TO  STUDY  THEM. 

them.  The  reagents  most  needed  are  the  three  acids, 
hydrochloric,  nitric,  and  sulphuric,  perhaps  also  a  little 
ammonia.  In  most  cases  it  is  best  to  use  the  strong  acids, 
but  often  these  diluted  with  an  equal  volume  of  water 
answer  every  purpose.  A  few  test-tubes  are  ulso  required, 
and  sometimes  a  porcelain  dish  or  casserole.  The  caution 
in  regard  to  chemical  reagents  already  mentioned  (p.  127) 
is  to  be  carefully  observed. 

Solubility  in  Acid. — The  question  as  to  whether  a  min- 
eral is  soluble  in  one  of  the  acids  named  is  often  of 
great  importance.  To  test  the  solubility  hydrochloric  acid 
is  generally  used,  except  with  metallic  sulphides  and  some 
other  minerals  containing  prominently  one  of  the  heavy 
metals  (lead,  copper,  silver,  etc.);  for  these  latter  nitric 
acid  is  usually  better.  The  mineral  should  in  general  be 
pulverized  as  finely  as  possible  in  the  agate  mortar  and  in- 
troduced into  a  large  test-tube,  some  acid  poured  on,  and 
the  whole  carefully  heated  over  the  Bunsen  flame,  the  tube 
being  shaken  gently  during  the  process. 

It  must  be  remembered  here  that  the  acid  fumes  in  the 
air  are  injurious  to  breathe  and  will  act  corrosively  upon 
surfaces  of  brass  in  the  neighborhood;  hence  such  tests 
can  only  be  tried  with  caution  unless  the  conveniences  of 
the  laboratory  are  at  hand. 

Various  results  may  be  noted  during  this  trial: 

A.  The  mineral  may  dissolve  quietly  with  or  without 
coloring  the  solution;  this  is  true,  for  example,  of  hema- 
tite, also  of  many  of  the  sulphates  and  phosphates. 

B.  There  may  be  a  bubbling  off  or  effervescence  of  a  gas. 
This  gas  is  usually  carbon  dioxide  or  carbonic-acid  gas 


THE   USE   OF  THE   BLOWPIPE.  155 

(CO,);  but  may  be  hydrogen  sulphide  or  sulphuretted  hy- 
drogen (HaS).  Also  chlorine  may  be  liberated,  or  reddish 
fumes  of  nitrogen. 

C.  There  may  be  a  separation  of  some  insoluble  sub- 
stance, as  sulphur,  silica,  etc.  These  points  will  now  be 
spoken  of  more  in  detail. 

Effervescence  with  Carbon  Dioxide. — This  resembles  the 
bubbling  observed,  for  example,  in  a  glass  of  soda-water, 
due  to  the  escape  of  this  same  gas  liberated  because  of  the 
relief  of  the  pressure  which  kept  it  dissolved  in  the  water 
in  the  tank.  This  is  an  easy  and  important  test  for  the 
carbonates.  Some  of  them  dissolve  in  cold  acid  and  even 
in  lumps  without  being  first  pulverized.  This  is  true  of 
calcite,  but  is  not  true  of  dolomite  and  siderite,  which  re- 
quire to  be  pulverized  or  heated,  or  both;  hence  this  is  used 
as  a  means  of  distinguishing  between  them.  The  carbo- 
nates of  copper  and  lead  should  be  tried  with  nitric  acid. 

Effervescence  with  Hydrogen  Sulphide.— Most  metallic 
minerals,  as  stated  above,  will  be  treated  with  nitric  acid, 
but  some  not  having  a  metallic  luster,  sphalerite  for  exam- 
ple, may  be  put  into  hydrochloric  acid.  In  this  case  the 
reaction  produces  the  gas  hydrogen  sulphide,  while  zinc 
chloride  goes  into  solution.  This  gas  bubbles  off  like  car- 
bon dioxide,  but  its  disagreeable  odor,  resembling  that  of 
rotten  eggs,  shows  at  once  what  it  is. 

Chlorine,  easily  detected  by  its  peculiar  odor,  is  given 
off  in  some  cases,  as  when  the  oxides  of  manganese  are 
heated  in  hydrochloric  acid. 

Nitrogen  Peroxide,  giving  a  peculiar  red  color  and  suffo- 
cating odor,  is  liberated  when  many  metallic  sulphides  (as 


I5e  MINERALS,  AND   HOW  TO  STUDY  THEM. 

chalcopyrite),  also  a  few  other  compounds,  as  cuprite,  are 
treated  with  nitric  acid. 

Separation  of  Sulphur. — A  number  of  sulphides,  as  for 
example  pyrite,  dissolve  in  nitric  acid  with  the  separation 
of  particles  of  sulphur  which  usually  cling  together  and 
float  on  the  liquid.  It  may  be  added  that  this  is  also  true 
of  chalcopyrite,  or  copper  pyrites,  but  this,  like  other  cop- 
per sulphides,  gives  a  green  solution  which  turns  a  deep 
fine  prussian  blue  when  ammonia  is  added  in  sufficient 
quantity  to  dissolve  the  precipitate  that  forms  at  first. 

Separation  of  Tin  Dioxide. — When  metallic  tin  is  treated 
with  nitric  acid,  tin  dioxide  (SnOJ  is  formed,  which  sepa- 
rates as  an  insoluble  white  powder. 

Separation  of  Silica. — A  number  of  silicates  dissolve  in 
hydrochloric  acid  with  the  separation  of  the  silica,  some- 
times as  a  powder,  sometimes  as  a  slimy  mass.  Other  sili- 
cates dissolve  entirely;  but  if  the  solution  is  gently  heated 
until  part  of  the  liquid  has  been  evaporated  off,  a  thick  jelly 
is  finally  formed,  so  that  the  test-tube  can  be  partially  in- 
verted without  its  flowing  out.  Such  silicates  are  said  to 
gelatinize  with  acid.  This  is  true  of  calamine  and  a  num- 
ber of  the  zeolites;  chabazite,  on  the  other  hand,  is  de- 
composed with  the  separation  of  slimy  silica. 

Difficultly -soluble  or  Insoluble  Minerals. — A  large  num- 
ber of  minerals,  even  when  pulverized,  dissolve  very  little 
or  not  at  all  in  strong  hot  acid.  Quartz  and  corundum,  for 
example;  also  the  silicates,  orthoclase,  topaz,  and  many 
others,  even  when  finely  pulverized  and  long  heated  in 
strong  acid,  are  not  at  all  or  only  very  slightly  attacked. 
The  question  whether  there  has  been  partial  solution  is  not 


THE    USE   OF   THE   BLOWPIPE.  157 

always  easy  to  answer,  but  can  be  decided  if  the  liquid  takes 
a  distinct  color,  or  more  fully  by  filtering  off  the  liquid 
from  the  undecomposed  mineral,  and  then  adding  to  it  a 
few  drops  of  ammonia,  which,  in  general,  will  cause  the 
bases  which  have  gone  into  solution  to  separate  as  precipi- 
tates. To  explain  the  various  ways — simple,  too,  many  of 
them — in  which  the  bases  present  in  the  solution  can  be 
identified  would  take  us  too  far  into  the  subject  of  Chemis- 
try. Do  not  forget,  however,  the  test  foi  copper  just  men- 
tioned (p.  156),  or  that  for  silver  given  on  an  earlier  page 
(p.  145).  Further,  attention  may  be  called  to  the  fact  that, 
as  a  test  for  sulphuric  acid  or  a  sulphate,  the  addition,  to  a 
solution  containing  them,  of  a  little  barium  chloride  will 
cause  a  heavy  white  precipitate  of  barium  sulphate  to 
form. 


158  MINERALS,  AND   HOW  TO   STUDY   THEM. 


CHAPTER  VII. 
DESCRIPTION  OF  MINERAL  SPECIES. 

THE  following  chapter  gives  descriptions  of  all  the  com- 
mon species  of  minerals,  with  remarks,  more  or  less  brief, 
about  many  of  those  which  are  rarer.  The  system  of 
classification  is  that  spoken  of  on  p.  120,  in  which  the 
different  compounds  of  the  same  metallic  element  are 
grouped  together.  The  Silicates,  however,  many  of  which 
are  complex  in  composition,  containing  more  than  one 
metal,  are,  with  the  exception  of  a  few  valuable  ores,  most 
conveniently  included  in  a  common  section  at  the  close  of 
the  chapter. 

The  several  characters  for  each  mineral  species  are 
enumerated  in  the  following  list : 

.Crystalline  system  ;  the  characteristic  angles  and  the 
common  form,  or  habit,  of  the  crystals;  also  thfe  structure 
of  the  crystalline  aggregates  and  massive  varieties. 

Cleavage;  also  fracture  and  tenacity. 

Hardness  (H.). 

Specific  gravity  (G.). 

Luster,  color,  streak,  degree  of  transparency. 

Other  physical  characters,  as  magnetism,  etc. 

Chemical  composition  and  blowpipe  characters.* 

*  These  last  are  also  called  pyrognostic  characters  because  depend- 
ing upon  the  application  of  heat  (nv/j,  fire);  this  word  is  often  con 
traded  to  Pyr. 


DESCRIPTION   OF    MINERAL   SPECIES.  159 

The  order  in  the  above  list  is  that  which  is  at  once  the 
most  convenient  and  scientific.  In  the  account  given  of 
each  species  in  the  following  pages,  however,  it  is  not  at- 
tempted to  adhere  to  this  order  strictly,  as  would  be  done 
in  an  advanced  scientific  work.  On  the  contrary,  so  far  as 
is  possible  in  the  brief  space  available,  the  aim  is  to  make 
this  account  readable  and  to  call  attention  especially  to 
the  characters  most  easy  or  most  important  to  remember. 

Further,  in  the  description  of  many  species  no  men- 
tion is  made  in  regard  to  certain  characters,  which  are 
relatively  unimportant  in  these  particular  cases.  Thus  if 
the  cleavage  is  not  mentioned,  it  is  because  it  is  either  not 
observed  or  too  imperfect  to  be  an  important  character. 
So,  too,  nearly  all  minerals  are  brittle,  hence  it  is  unneces- 
sary to  repeat  this  word  in  each  case;  but  if  the  mineral  is 
not  brittle  but  malleable  or  sectile,  this  is  stated  and  to  be 
carefully  noted.  Again,  if  the  streak  is  not  given,  it  is  to 
be  understood  to  be  white  or  nearly  white,  like  that  of  most 
non-metallic  minerals,  even  when  the  mineral  itself  in  the 
mass  has  a  deep  color.  Also  all  minerals  if  having  a  metallic 
luster  ire  opaque.  The  localities  of  the  species  are  men- 
tioned, if  at  all,  very  briefly. 

The  student  will  find  it  easier  to  remember  the  charac- 
ters of  the  different  minerals,  and  a 'help  in  other  ways,  if. 
after  studying  the  descriptions  in  the  book  and  comparing 
them  with  such  specimens  as  he  has  access  to,  he  will  make 
a  brief  tabular  list  of  the  characters  for  each  species,  some- 
thing like  that  on  the  following  page. 

It  is  very  easy  to  arrange  a  note-book  (conveniently  of 
the  square  letter  size)  for  this  purpose  by  ruling  a  series  d* 


160 


MINERALS,  AND    HOW   TO    STUDY   THEM. 


Diamond. 

Graphite. 

Galena. 

Sphalerite. 

Cryst.systemA 

common  form 

Cryst  &  mass 

Isometric 
octahedron 

Hexagonal 
tabular 

Foliated 

Isometric 
cube 
Granular- 
cleavable 

Isometric 
tetrahedral 
Granular- 
cleavable 

Cleavage  

Octahedral 

Basal 

Cubic 

Dodecahedral 

Hardness,  etc. 

10! 

1-2! 
flexible 

2.5-« 

3.5-4 

Gravity  

3.5 

2.2 

7.51 

4 

Luster  

Adamantine 

Metallic 

Metallic 

Ruinous 

Color  

Colorless,  yel'w 

Black 

Lead-gray 

Yellow,  brown, 
black,  etc. 

Str&ak  

White 

Black 

Dark  gray 

White  to  brown 

Comp  

Carbon 

Carbon 

PbS 

ZnS 

Pyr.  etc.  

Infusible 

Infusible 

Easily  fusible 

Infusible 

parallel  vertical  columns,  and  the  trouble  of  writing  the 
list  of  characters  over  each  time  may  be  avoided  if  they 
are  written  on  the  edge  of  the  first  left  page  and  the 
corresponding  strip  from  a  sufficient  number  of  the  sheets 
following  neatly  cut  off.  A  little  contraction  of  some 
common  words  will  save  space :  hardness  is  often  indicated 
by  the  letter  H.;  specific  gravity  by  G.;  yellow  may  be 
written  yw,  and  so  on.  When  a  character  is  particularly 
important  it  may  be  underscored  or  followed  by  an  exclama- 
tion point.  It  is  not  worth  while  to  repeat  in  tabular  form 
the  entire  description  in  the  text;  a  little  experience  will 
soon  show  how  much  may  be  advantageously  written  down. 
It  will  also  be  a  useful  exercise  to  fill  out  a  similai 
column,  so  far  as  the  individual  case  allows,  for  any  specie» 
from  the  specimen  itself,  and  then  it  may  be  compared 
with  the  description  in  the  book,  or  the  list  in  the  stu~ 
dent's  note-book  made  out  from  the  book.  If  the  species 
was  not  known  at  first,  this  list  of  characters  will  often 
suffice  to  enable  the  student  to  determine  it. 


DESCRIPTION  OF   MfifERAL   SPECiES.  161 

It  is  not  necessary  to  learn  by  sheer  effort  of  memory  all 
the  characters  from  the  book  at  once;  this  would  be  diffi- 
cult and  tiresome;  the  most  important  can  be  learned  (and 
first  the  chemical  composition),  while  the  knowledge  of 
most  of  the  physical  characters  is  rather  to  be  acquired 
gradually  by  the  repeated  handling  of  the  specimens 
themselves. 

The  following  is  a  summary  of  the  species  included  in 
the  pages  which  follow,*  arranged,  except  for  the  silicates, 
under  the  prominent  element  of  which  they  are  com- 
pounds. Muny  other  species  are  mentioned  briefly  in  the 
text,  though  not  included  here.  The  student  should  read 
again  the  brief  statements  in  regard  to  the  classification  of 
the  chemical  elements  and  the  prominent  groups  of 
chemical  compounds  given  on  pp.  109  to  116. 

It  may  be  interesting  here  to  recall  the  old  alchemistic 
method  of  designating  the  chief  metals  by  referring  them 
to  one  of  tin  members  of  the  Solar  System,  as  follows :  the 
Sun,  gold;  the  Moon,  silver;  the  planet  Mercury,  mercury; 
Venus,  copper  ;  Mars,  iron  ;  Jupiter,  tin  ;  Saturn,  lead. 
Other  prominent  metals  are  platinum,  zinc,  and  aluminium. 

CARBON:  Diamond. 

Graphite. 

SULPHUR:          Native  Sulphur. 
HYDROGEN:       ice  (and  water). 

*  A  list  is  given  in  the  Appendix  of  those  of  the  species  here 
enumerated  which  it  is  most  important  for  the  student  to  have  in  hie 
collection. 


162 


MINERALS,    AND  HOW   TO   STUDY  THEM. 


ARSENIC  :  Native  Arsenic. 

Realgar  and  Orpiment,  Arsenic  sulphide* 
ANTIMONY  :        Native  Antimony. 

Stibnite,  Antimony  sulphide. 
BISMUTH:  Native  Bismuth. 

MOLYBDENUM:  Molybdenite,  Molybdenum  sulphide. 
GOLD:  Native  Gold. 

Sylvanite,  Gold  telluride. 
PLATINUM:         Native  Platinum. 
SILVER:  Native  Silver. 

Argentite,  Silver  sulphide. 

Pyrargyrite,  Sulphide  of  silver  and  ant: 
mony. 

Proustite,  Sulphide  of  silver  and  arsenic. 

Cerargyrite,  Silver  chloride. 
MERCURY:          Native  Mercury. 

Cinnabar,  Mercury  sulphide. 
COPPER:  Native  Copper. 

Chalcocite,  Copper  sulphide. 

Bornite  and    Chalcopyrite,   Sulphides  o: 
copper  and  iron. 

Tetrahedrite,  Sulphide  of  antimony  anc 
copper. 

Cuprite,  cuprous  oxide. 

Malachite  and  Azurite,  Carbonates  of  co[ 
per. 

Dioptase  and  Chrysocolla,  Silicates  of  cop- 
per. 
LEAD:  Native  Lead. 

Galena,  Lead  sulchide 


DESCRIPTION   OF   MINERAL   SPECIES. 


163 


Jamesonite  and  Bournonite,  Sulphides  of 
antimony  and  lead. 

Pyromorphite,  Lead  phosphate. 

Mimetite,  Lead  arsenate. 

Vanadinite,  Lead  vanadate. 

Cerussite,  Lead  carbonate. 

Anglesite,  Lead  sulphate. 

Also  Crocoite,  Lead  chromate;  Wulfenite, 

Lead  molybdate,  etc. 
TIN  :  Cassiterite,  Tin  dioxide. 

TITANIUM:         Rutile;  also  Octahedrite  and  Brookite,  all 

alike  Titanium  dioxide,  TiO,. 
URANIUM:         Uraninite. 

Torbernite,  Autunite,  Uranium  phosphates. 
IRON:  Native  Iron. 

Pyrrhotite,  Iron  sulphide. 

Pyrite  and  Marcasite,  Iron  disulphide. 

Arsenopyrite,  Iron  sulph -arsenide. 

Hematite,  Iron  sesquioxide. 

Magnetite,  Magnetic  iron  oxide. 

Franklinite,  Iron-zinc-manganese  oxide. 

Chromite,  Iron-chromium  oxide. 

Limonite,  Hydrated  iron  oxide. 

Siderite,  Iron  carbonate. 

Also  Columbite,  Iron  niobate  (columbate) 
and  Wolframite,  Iron  tungstate ;  Triphy» 
lite,  Phosphate  of  iron  and  lithium. 
NICKEL:  Millerite,  Nickel  sulphide. 

Niccolite,  Nickel  arsenide. 

Genthite  and  Garnierite,  Nickel  silicates. 


164 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


COBALT:  Linnaeite,  Cobalt  sulphide. 

Smaltite  and  Cobaltite,  Arsenides  of  co- 
balt. 

Erythrite,  Cobalt  arsenate. 

MANGANESE:     Pyrolusite  and  Manganite,  Oxides  of  man- 
ganese. 

Khodonite,  Manganese  silicate. 
Rhodochrosite,  Manganese  carbonate. 
ZINC:  Sphalerite,  Zinc  sulphide. 

Zincite,  Zinc  oxide. 

Willemite  and  Calamine,  Zinc  silicates. 
Smithsonite,  Zinc  carbonate. 
ALUMINIUM:      Corundum,  Aluminium  oxide. 

Spinel,  Oxide  of  magnesium  and  alumin- 
ium. 

Cryolite,  Fluoride  of  aluminium  and   so- 
dium. 

Turqnois  and  Wavellite,  Aluminium  phos- 
phates;    Amblygonite,     Phosphate    of 
aluminium  and  lithium. 
CALCIUM:  Fluorite,  Calcium  fluoride. 

Calcite  and  Aragonite,  Calcium  carbonates. 
Apatite,  Calcium  phosphate. 
Anhydrite,  Calcium  sulphate. 
Gypsum,  Hydrated  calcium  sulphate. 
Scheelite,  Calcium  tungstate. 
MAGNESIUM:      Brucite,  Hydrated  magnesium  oxide. 

Magnesite  and  Dolomite,  Magnesium  car- 
bonates. 
Boracite,  Magnesium  borate. 


DESCRIPTION   OF   MINERAL  SPECIES. 


165 


BARIUM:  Barite,  Barium  sulphate. 

VYitherite,  Barium  carbonate. 
STRONTIUM  :       Celestite,  Strontium  sulphate. 

Strontianite,  Strontium  carbonate. 
SODIUM  and   POTASSIUM:   Halite  or  Rock  Salt,  Sodium 

chloride. 

Borax,  Sodium  borate. 
Sylvite,  Potassium  chloride. 
SILICON  :  Quartz,  Silicon  dioxide. 

Opal,  Hydrated  silicon  dioxide. 

SILICATES  :  *  Feldspars :  Orthoclase  (and  Microcline), 
Albite,  Anorthite  ;  also  Oligoclase, 
Labradorite. 

Pyroxene  (Diopside,  Salite,  Augite,  etc.). 
Amphibole     or    Hornblende    (Tremolite, 

Actinolite,  Asbestus,  etc.). 
Beryl. 

Garnet  (Grossularite,  Almandite,  etc.) 
Micas:    Muscovite,    Biotite,    Phlogopite, 

Lepidolite. 

Chlorites :  Clinochlore,  etc. 
Chrysolite. 
Zircon. 
Scapolite. 
Vesuvianite. 
Epidote. 
Tourmaline. 
Topaz. 

*  The  composition  of  the  following  minerals  is  in  many  cases  too 
complex  to  be  given  briefly  here. 


166  MINERALS,  AND    HOW  TO    STUDY   THEM. 

Titanite  or  Sphene. 
Andalusite,  Sillimanite  and  Cyanite. 
Staurolite. 
Talc. 

Serpentine. 
Datolite. 
Prehnite. 
Apophyllite. 
Pectolite. 

Zeolites :  Thomsonite,  Natrolite,  Aualcite, 
Chabazite,  Stilbite,  Heulandite. 


CARBON. 
Diamond.     Carbon,  C. 

The  DIAMOND  is  usually  found  in  distinct  isolated  crys- 
tals, most  of  them  very  small,  but  sometimes  as  large  as  a 
robin's  egg  or  even  larger.  The  crystals  are  commonly 
octahedrons,  though  less  often  some  of  the  other  forms  of 
the  isometric  system  (p.  22)  are  observed.  The  natural 
crystals  before  cutting — "  rough  diamonds  "  they  are  called 
— frequently  have  rounded  edges  and  curved  faces,  or  the 
faces  show  little  pits  like  the  etchings  spoken  of  on  p.  64. 
This  is  illustrated  in  Fig.  157,  while  Fig.  158  shows  a  hex- 
octahedron  with  convex  faces.  There  are  also  forms  with 
irregular  structure,  occasionally  as  round  as  peas,  and  one 
peculiar  kind  is  massive  and  dull  black  in  color. 

The  crystals  have  perfect  cleavage  parallel  to  the  octa- 
hedral faces,  which  the  lapidary  makes  use  of  to  bring  a 
stone  into  the  form  best  suited  for  cutting.  The  hardness 


DESCRIPTION    OF    MINERAL   SPECIES.  167 

is  10,  or  higher  than  that  of  any  other  species,  and  the 
specific  gravity  is  also  high,  3.5  (see  p.  84).  The  luster  is 
very  brilliant  and  of  tho  peculiar  character  named  (from 
this  species)  adamantine;  the  brilliancy  of  the  diamond, 
however,  is  much  greater  when  cut  with  many  facets  than 
in  the  natural  crystals.  The  most  highly  prized  stones 
are  colorless  and  clear  as  water  (then  said  to  be  "  of  the 
first  water") ;  a  pale  yellow  color  is  very  common,  and  some- 
times other  colors,  in  pale  shades,  as  green,  pink,  and  blue, 
are  observed;  rarely  it  is  black  and  dull. 
The  diamond  consists  of  pure  carbon,  and  has  thus  the 

157.  158. 


same  composition  as  a  piece  of  charcoal.  It  is  infusible  as 
is  charcoal,  and  is  not  acted  upon  by  acids;  but  it  is  unlike 
charcoal  in  that  it  does  not  burn.  When  heated  very  hot, 
however,  as  in  the  electric  arc,  it  is  slowly  consumed,  form- 
ing, like  burning  charcoal,  carbon  dioxide  or  carbonic-acid 
gas  (CO,).  Heated  out  of  contact  with  the  oxygen  of  the 
air  it  is  converted  into  a  mass  resembling  coke. 

The  diamond  has  been  found  mostly  in  gravel  deposits, 
or  the  rocks  formed  by  their  consolidation,  and  but  little  is 
known  about  its  real  home  or  the  way  it  was  made.  For- 
merly it  was  obtained  in  great  quantities  in  India;  later 
Brazil  afforded  many  of  the  gems,  but  both  these  countries 


168  MINERALS,  AND   HOW   TO   STUDY   THEM. 

now  yield  comparatively  few.  The  great  region  which  pro- 
duces the  diamond  at  the  present  time  is  in  South  Africa, 
some  eight  hundred  miles  from  Cape  Town,  where  it  occurs 
along  the  Vaal  River,  and  in  larger  quantities  especially  in 
the  neighborhood  of  Kirnherly,  in  peculiar  oval  regions 
called  "  pans."  Here  the  diamond  mines  have  been  worked 
for  about  twenty-five  years  and  a  vast  number  of  stones 
have  been  found  and  brought  to  market.  Think  of  more 
than  eight  tons  of  diamonds  obtained  during  this  time! 

Everybody  knows  of  the  use  of  the  diamond  for  jewelry, 
for  which  its  brilliancy,  hardness,  and  comparative  rarity 
peculiarly  fit  it.  Many  of  the  great  diamonds  of  the  world 
have  a  long  and  fascinating  history  of  their  own,  which 
would  fill  a  large  volume  if  all  were  told  of  the  way  in  which 
they  have  repeatedly  changed  hands  until  they  have  come 
into  the  possession  of  royalty,  as  the  famous  "  Kohinoor  " 
among  the  crown  jewels  of  England,  or  the  "  Florentine  " 
of  Austria  and  the  great  "  Orlov "  diamond  of  Russia. 
Diamonds  are  also  used  for  cutting  glass  and,  in  the  form  of 
powder,  in  grinding  diamonds  and  other  hard  gems.  The 
black  coal-like  diamonds,  set  in  a  collar  and  rotated  rapidly 
by  machinery,  as  a  diamond  drill,  cut  quickly  through  the 
hardest  rocks,  leaving  a  core  behind,  which  is  raised  at  in- 
tervals; a  well-boring  is  thus  easily  made. 

Graphite  or  Plumbago.     Carbon,  C. 

GRAPHITE,  or  Plumbago  as  it  is  often  called,  is  usually 
found  in  massive  forms  which  may  be  separated  easily  into 
thin  leaves  or  plates  and  hence  are  said  to  be  foliated; 
sometimes  also  it  is  finely  granular  and  compact.  Rarely 


DESCRIPTION    OF   MINERAL   SPECIES.  169 

the  plates  are  distinct  and  separate  and  show  a  regular  six- 
sided  outline,  whence  it  is  referred  to  the  hexagonal  system; 
it  is  then  seen  to  ha\e  perfect  basal  cleavage. 

It  is  sectile  and  so  soft  as  to  make  a  mark  on  paper  and 
to  feel  greasy  to  the  hand  (  H.  =  1  to  2),  and  its  specific 
gravity  is  only  2.2.  It  has  a  metallic  luster  and  an  iron- 
black  or  steel-gray  color  and  streak;  it  is  perfectly  opaque. 

Graphite  has  the  same  composition  as  the  diamond,  con- 
sisting also  of  nearly  pure  carbon;  it  is,  however,  a  differ- 
ent substance  in  its  physical  characters  and  is  hence  a  dis- 
tinct mineral.  Note  that  they  differ  in  crystalline  form; 
also  the  diamond  is  hard  and  heavy,  while  graphite  is  soft 
and  light. 

It  is  also  infusible  like  the  diamond,  and  is  not  at- 
tacked by  acids,  but  may  be  converted  into  carbon  dioxide 
(C02)  by  heating  to  a  very  high  temperature  in  the  air. 

Graphite  is  commonly  found  in  the  crystalline  rocks 
called  gneiss,  sometimes  scattered  in  scales,  but  occasionally 
in  large  beds  that  can  be  mined;  it  is  also  found  in  scales 
in  crystalline  limestone,  and  is  often  formed  in  an  iron  fur- 
nace. It  is  largely  mined  at  Ticonderoga,  N.  Y. ;  also  in 
Eastern  Siberia,  and  in  Ceylon. 

Graphite  is  the  so-called  black  lead  of  our  "  lead-pencils  " 
(but  it  is  only  like  lead  in  its  color),  and  would  be  mined  for 
this  purpose  if  for  no  other.  It  is  used  as  an  excellent 
lubricator  because  of  its  smooth  soapy  character  when  pul- 
verized ;  also,  mixed  with  clay,  for  making  crucibles  because 
it  is  infusible  and  not  affected  by  the  heat  of  an  ordinary 
furnace;  in  electroplating  because  it  is  a  conductor  of  elec- 
tricity. 


170  MINERALS,  AND   HOW   TO   STUDY   THEM. 

CARBON  is  also  the  element  which  forms  the  essential  part 
of  the  different  kinds  of  coal  and  of  mineral  oil  or  petro- 
leum. 

Anthracite,  the  coal  of  eastern  Pennsylvania,  contains  85 
to  95  per  cent  of  carbon  and  has  a  bright  shiny  surface  and 
conchoidal  fracture;  it  burns  with  a  pale  feeble  flame 
without  smoke. 

Bituminous  coal  is  black  to  dark  brown  in  color,  often 
dull  and  with  a  pitchy  luster;  it  contains  less  carbon  than 
anthracite  (usually  75  per  cent)  and  more  hydrogen  and 
oxygen;  it  burns  with  a  yellow  smoky  flame.  Brown  coal, 
or  lignite,  has  a  brown  color,  dull  luster,  often  retains  the 
structure  of  the  original  wood  and  contains  still  less  carbon, 
sometimes  only  50  per  cent.  These  different  kinds  of 
coal  and  others  related  to  them,  though  of  great  economic 
value,  are  not  properly  mineral  species,  since  they  have  no 
definite  chemical  composition.  The  same  remark  applies 
to  asphaltum,  bitumen,  mineral  wax  or  ozocerite,  the  many 
kinds  of  mineral  resins  including  amber,  and  finally  min- 
eral oil  or  petroleum,  all  of  which  consist  chiefly  of  carbon. 

The  element  carbon  is  also  present  in  the  large  group  of 
minerals  called  carbonates,  of  which  calcite,  including  com- 
mon limestone,  is  much  the  most  important. 

SULPHUR. 

Native  Sulphur.     S. 

SULPHUR  is  another  of  the  chemical  elements  occurring 
in  nature.  It  is  found  in  crystals  of  the  orthorhombic 
system;  a  common  form  is  an  acute  rhombic  pyramid  (Fig. 
159)  with  terminal  angles  of  106£°  and  85°.  Figs.  160, 161 


DESCRIPTION    OF   MINERAL   SPECIES. 


171 


are  also  common  forms.  It  also  occurs  in  masses  and  in 
powder.  It  is  soft  (H.  =  l.o  to  2.5)  and,  though  brittle  un- 
der the  blow  of  a  hammer,  is  easily  cut  by  the  knife;  the 
specific  gravity  is  about  2.  It  has  a  resinous  luster  and  a 
bright  sulphur-yellow  color  and  streak.  The  crystals  are 
often  clear  and  transparent.  It  consists  of  pure  sulphur, 
and  is  remarkable  among  minerals  because  when  heated  it 
takes  fire  and  burns  with  a  pale  blue  flame,  giving  a  gas 
(sulphur  dioxide,  S02)  which  has  a  very  suffocating  odor 
familiar  to  all  who  use  sulphur  matches. 

Sulphur  is  for  the  most  part  found  in  volcanic  regions,  as 
in  Sicily  and  the  Sandwich  Islands;  also  in  beds  associated 


159. 


160. 


161. 


with  gypsum.  It  is  used  for  making  sulphur  matches;  it 
is  one  of  the  three  substances  of  which  gunpowder  is 
made  (with  charcoal  and  niter) ;  it  is  used  in  preparing  the 
rubber  gum  for  overshoes  and  other  purposes;  also  in 
making  sulphuric  acid  and  in  other  ways.  It  is  in  fact  a 
most  important  mineral. 

Sulphur  also  occurs  abundantly  in  nature,  not  as  an  ele- 
ment, but  in  combination  with  the  metals  forming  the  very 
large  and  important  class  of  sulphides,  as  lead  sulphide, 
PbS,  the  mineral  galena.  It  also  forms  the  acid,  sulphuric 


172  MINERALS,  AND    HOW  TO   STUDY   THEM. 

acid,  HaS04,  the  salts  of  which  are  the  important  class 
of  sulphates,  as  barium  sulphate,  BaS04,  the  mineral 
barite. 

Ice.     Hydrogen  oxide,  H20. 

Although  it  cannot  be  preserved  in  a  mineral  cabinet, 
ICE,  the  solid  form  of  water,  is  as  truly  a  mineral  as 
diamond  or  quartz.  It  occurs  in  crystalline  forms  of  the 
hexagonal  type,  often  of  great  complexity  and  beauty,  as 
seen  in  snow-crystals.  These,  as  stated  on  p.  17,  are 
formed  in  the  atmosphere  direct  from  the  water  vapor. 
Some  of  the  forms  are  shown  in  figure  on  p.  17.  The 
ice-grains  that  make  the  pellets  of  hail,  not  infrequently 
occurring  with  summer  thunder-storms,  are  also  occa- 
sionally in  clusters  of  crystals,  somewhat  resembling 
the  hexagonal  pyramids  of  quartz,  though  this  is  the 
exception;  generally  there  is  simply  a  concentric  con- 
cretionary structure.  The  ice  of  the  pools  and  ponds  is 
always  crystalline,  though  it  is  usually  only  in  the  first 
stages  of  the  process  of  freezing  that  the  crystals  are 
separately  visible.  This  process  of  solidification  goes  on, 
as  every  one  knows,  at  a  temperature  of  32°  Fahrenheit 
(0°  Centigrade).  The  hardness  of  ice  near  the  freezing- 
point  is  1.5,  but  this  increases  at  lower  temperatures.  The 
specific  gravity  is  about  0.92,  so  that  it  floats  in  the  water 
with  a  little  more  than  nine  tenths  of  its  bulk  submerged. 
Water  expands,  therefore,  largely  on  freezing  and  exerts  a 
great  force  on  confining  surfaces.  One  consequence  Oi  this 
is  the  breaking  of  vessels,  water-pipes,  etc.,  when  the  water 
they  contain  is  frozen.  In  nature  ice  is  on  account  of  this 


DESCRIPTION   OF   MINERAL   SPECIES.  173 

property  a  powerful  agent  in  pulling  rocks  to  pieces,  the 
water  creeping  into  the  cracks,  especially  into  the  narrow 
ones,  by  capillarity,  and  when  it  solidifies  the  rock  masses 
are  slowly  but  surely  wedged  apart. 

Water  consists  chemically  of  hydrogen  and  oxygen, 
combined  in  the  ratio  of  2  :  1  by  volume,  or  11.1  :  88.9  by 

weight. 

TELLURIUM. 

Carbon  in  its  two  forms,  the  diamond  and  graphite, 
and  sulphur  belong,  as  was  stated  on  p.  102,  to  the  non- 
metals  among  the  chemical  elements.  Intermediate  be- 
tween them  and  the  true  metals,  like  gold  and  silver, 
come  several  elements  which  occur  in  nature,  namely, 
tellurium,  arsenic,  antimony,  and  bismuth. 

TELLURIUM  has  a  bright  tin-white  color  and  a  metallic 
luster,  though,  unlike  the  true  metals,  it  is  rather  brittle; 
it  is  occasionally  found  in  Colorado.  This  element  is  of 
little  economic  importance,  but  is  interesting  because  it  is 
the  only  one  with  which  gold  occurs  combined  in  nature, 
in  some  of  the  rare  minerals  called  tellurides  (see  p.  181). 

ARSENIC. 

ARSENIC  is  found  occasionally  as  a  mineral  and  then 
called  NATIVE  ARSENIC.  It  has  a  metallic  luster  and  tin- 
white  color,  but  soon  tarnishes  on  the  surface  to  a  dull 
dark  gray;  it  is  also  brittle.  It  generally  occurs  showing 
a  fine  granular  structure  when  fractured,  and  the  masses 
commonly  have  a  reniform  or  botryoidal  surface. 

Arsenic  is  used  with  copper  and  tin  to  form  the  alloy 
called  speculum  metal,  useful  for  metallic  mirrors  because 


174  MINERALS,  AND   HOW   TO   STUDY   THEM. 

of  the  brilliant  surface  it  takes  when  polished.  The  lead 
employed  for  making  shot  contains  a  small  amount  of 
arsenic.  The  compounds  of  arsenic  find  various  uses,  as 
pigments,  (sulphide);  as  a  preservative;  a  poison  for  in- 
sects (white  arsenic  and  Paris  green);  also  in  dyeing, 
medicine,  etc. 

Kealgar,  Orpiment.     Sulphides  of  Arsenic. 

Two  important  but  rather  rare  minerals  containing  ar- 
senic are  the  sulphides,  Realgar,  AsS,  and  Orpimeut,  AsaS8. 

EEALGAR  is  found  in  transparent  monoclinic  crystals 
and  massive  forms  of  a  beautiful  aurora-red  color.  It  is 
soft  and  sectile  (H.  =  1.5-2)  and  has  a  specific  gravity  of 
3.5;  the  luster  is  resinous.  Its  composition  is  AsS,  or 
arsenic  monosulphide,  which  gives  the  percentage  com- 
position: Sulphur  29.9,  arsenic  70.1  =  100. 

ORPIMENT,  named  from  the  Latin  auripigmentum,  or 
Gold  pigment  (also  called  King's  yellow),  is  of  a  beautiful 
golden  yellow.  It  is  generally  found  in  masses  showing  a 
foliated  structure  and  with  one  perfect  cleavage  so  that  it 
can  be  split  off  into  thin  flexible  leaves.  Distinct  crystals 
of  orpiment  are  very  rare;  they  belong  to  the  ortho- 
rhombic  system.  It  is  soft  (H.  =  1.5-2),  sectile,  and  the 
specific  gravity  is  about  3.5. 

The  composition  is  AsaS3,  or  arsenic  trisulphide,  which 
gives:  Sulphur  39.0,  arsenic  61.0  —  100.  Its  behavior  in 
the  closed  and  open  tubes  is  mentioned  on  p.  150;  on 
charcoal  it  is  all  volatilized,  giving  the  characteristic  garlic 
odor  of  arsenic  and  white  fumes  of  the  oxide  (Asa08). 

Arsenic  is  present  in  a  great  many  other  minerals.     It 


DESCRIPTION   OF   MINERAL   SPECIES.  175 

forms  with  the  metals  a  series  of  compounds  called  arsen- 
ides, of  which  arsenopyrite  and  cobaltite  are  examples. 
It  forms  with  sulphur  a  number  of  compounds  of  the 
metals,  as  proustite.  There  are  also  a  series  of  salts  called 
ar senates,  one  of  which  is  the  lead  arsenate  mimetite;  an- 
other is  the  cobalt  arsenate,  erythrite. 

White  arsenic,  or  the  "arsenic  of  the  druggist,"  is  the 
oxide,  AsaOs,  which  occasionally  occurs  as  a  mineral  (then 
called  ARSENOLITE).  It  is  formed  whenever  metallic  ar- 
senic or  an  arsenide  is  roasted  in  the  air.  In  the  open 
tube  it  is  often  obtained  in  spangling  octahedral  crystals 
(see  pp.  17  and  150). 

ANTIMONY. 

ANTIMONY,  like  bismuth,  is  usually  included  among  the 
metals,  for  it  has  a  high  metallic  luster,  although  its 
structure  is  crystalline  and  it  is  quite  brittle.  It  is  a  very 
easily  fusible  metal  and  is  useful  in  the  arts  because  of  the 
alloys  which  it  forms  with  lead  and  tin,  to  which  it  imparts 
greater  hardness  and  durability.  Thus  type-metal  is  an 
alloy  of  one  part  of  antimony  to  three  or  four  of  lead. 
Britannia  metal,  often  used  as  the  base  of  plated  silver- 
ware, is  an  alloy  of  antimony  with  brass,  tin,  and  lead. 
Babbitt  metal,  used  for  bearings,  is  another  alloy  of  anti- 
mony with  tin  and  copper.  Tartar  emetic,  used  in  medi- 
cine, is  tartrate  of  antimony  and  potassium. 

Native  Antimony,  Sb. 

NATIVE  ANTIMONY  is  a  bright  tin-white  mineral  with 
metallic  luster,  and  commonly  showing  brilliant  cleavage 


176  MINERALS,  AND    HOW   TO   STUDY    THEM. 

surfaces;  rhombohedral  crystals  are  rare.  Its  hardness  is 
3  to  3.5,  and  the  specific  gravity  6.7.  It  is  not  a  common 
mineral,  but  is  found  in  New  Brunswick  in  some  quantity, 
in  California  and  elsewhere.  Heated  on  charcoal  it  fuses 
and  goes  off  entirely  in  white  fumes  of  the  trioxide,  SbaO,; 
in  the  open  tube  dense  white  fumes  of  this  oxide  are  also 
deposited  (see  p.  151). 

Stibnite,  or  Antimony  Glance.    Antimony  Sulphide,  Sb2S,. 

STIBNITE,  the  sulphide  of  antimony,  is  its  commonest 
and  most  important  ore.  It  is.  found  in  prismatic  crystals 
of  the  orthorhombic  system,  often  spear-shaped  at  the  ends 
(Fig.  162).  These  crystals  are  frequently  acicular  and 
arranged  in  radiating  groups,  or  again  they 
may  be  very  large;  the  mines  in  Japan  have 
afforded  specimens  magnificent  in  size  and 
brilliancy  of  luster. 

The  crystals  have  very  perfect  cleavage, 
parallel  to  one  vertical  edge,  and  the  surfaces 
formed  by  this  are  smooth  and  highly  polished. 
Besides  the  prismatic  crystals,  stibnite  also 
occurs  in  massive  forms,  generally  columnar  in 
structure  and  then  also  showing  the  perfect 
cleavage  ;  but  also  sometimes  compact  and 
granular  and  then  the  cleavage  is  not  apparent. 
The  hardness  is  only  2,  so  that  it  is  scratched 
by  the  nail  and  leaves  a  mark  on  paper;  it  is  quite  sectile. 
It  is  not,  however,  to  be  confounded  with  graphite,  which 
is  much  more  soft  and  greasy  in  feel  and  marks  the  paper 
without  the  slightest  tendency  to  tear  it.  The  specific 


DESCRIPTION   OP  MINERAL  SPECIES.  177 

gravity  of  stibnite  is  about  4.6.  The  luster  is  metallic  and 
on  a  fresh  surface — particularly  a  cleavage  surface — it  is 
very  brilliant,  as  already  noted.  The  color  is  a  bluish 
gray,  but  less  blue  than  galena,  with  which  it  is  sometimes 
confounded  (but  note  the  difference  in  cleavage);  the 
streak  is  nearly  black. 

Stibnite  is  the  sulphide  of  antimony  (antimony  tri- 
sulphide),  Sb,Ss;  this  gives  the  percentage  composition: 
Sulphur  28.6,  antimony  71.4  =  100.  Heated  on  charcoal 
it  fuses  very  easily  and  gives  off  fumes  of  the  oxide  of 
antimony  (Sb203),  which  form  a  thick  coating  at  a  little 
distance;  after  a  few  moments  the  fragment  is  entirely 
volatilized.  If  the  reducing  flame  is  thrown  for  a  moment 
on  the  coating,  it  is  burned  off  with  a  greenish-blue  flame. 
In  the  open  tube,  heated  slowly,  the  same  dense  deposit 
or  sublimate  is  formed  in  the  cold  portion ;  this  is  powdery 
and  not  readily  volatile  like  the  somewhat  similar  white 
oxide  of  arsenic.  In  the  closed  tube  a  dark  red  sublimate 
of  antimony  oxysulphide  is  formed  (cf.  p.  151). 

Antimony  also  enters  into  a  number  of  other  minerals, 
as  pyrargyrite,  or  dark  red  ruby-silver;  also  tetrahedrite,  or 
gray  copper,  jamesonite,  bournonite,  etc.  These  are  further 
mentioned  under  the  metals  of  which  they  are  compounds; 
for  a  description  of  the  other  related  minerals  reference 
must  be  made  to  larger  works  on  mineralogy. 

BISMUTH. 

BISMUTH  is  silver-white  in  color  with  a  reddish  tinge 
and  has  a  bright  metallic  luster;  it  is  rather  brittle  and 
shows  a  crystalline  structure  with  perfec4  cleavages;  it  is. 


178  MINERALS,  AND   HOW  TO    STUDY   THEM. 

however,  nearer  to  the  true  metals  than  either  arsenic  or 
antimony.  Native  bismuth  is  a  rare  mineral,  and  its  com- 
pounds, chiefly  among  the  sulphides,  are  also  too  rare  to 
be  particularly  mentioned  here.  The  sulphide  of  bismuth, 
or  bismuthinite,  resembles  stibnite  rather  closely  in  phys- 
ical characters. 

Bismuth  is  an  even  more  fusible  metal  than  antimony, 
and  the  alloys  which  it  forms  are  remarkable  for  their  low 
melting-points;  an  alloy  of  bismuth  with  lead  and  tin 
fuses  at  a  temperature  below  that  of  boiling  water;  an- 
other alloy  of  the  same  metals  in  different  proportions  is 
used  as  a  kind  of  solder.  Some  bismuth  alloys  have  the 
curious  property  of  contracting  instead  of  expanding  with 
heat.  Bismuth  is  also  employed  in  medicine  in  the  form 
of  the  subnitrate;  another  compound  is  used  as  a  cosmetic; 
other  uses  are  in  calico-printing,  to  give  luster  to  porcelain, 

etc. 

MOLYBDENUM. 

Molybdenite.  Molybdenum  sulphide,  MoS,. 
MOLYBDENITE  is  the  sulphide  of  the  rare  element  mo- 
lybdenum. It  is  not  a  common  mineral,  but  is  found  in 
small  quantities  in  a  good  many  localities,  chiefly  in  crys- 
talline rocks  like  gneiss.  Like  graphite,  which  it  much 
resembles,  it  occurs  in  foliated  masses  or  in  crystalline 
plates  having  a  hexagonal  outline;  rarely  in  distinct  hex- 
agonal crystals.  It  is  also  very  soft  (H.  =  1-1.5)  with  a 
soapy  feel  and  leaves  a  trace  on  paper.  It  has  a  bluish- 
black  color  and  metallic  luster.  The  color,  however,  is 
distinctly  bluer  and  the  specific  gravity  (G.  =  4.7)  is 
higher  than  that  of  graphite. 


DESCRIPTION   OF   MINERAL   SPECIES.  179 

The  composition  of  Molybdenum  disulphide,  MoS9, 
gives:  Sulphur  40.0,  molybdenum  60.0  =  100.  Heated  in 
the  open  tube  or  on  charcoal  it  gives  off  strong  sulphur 
fumes  and  yields  a  deposit,  which  is  pale  yellow  or  white, 
of  molybdic  oxide;  this  coating  on  charcoal,  if  touched 
with  an  intermittent  blowpipe  flame  (reducing  flame)  be- 
comes a  bright  blue  (see  p.  146). 

Molybdenum  also  occurs  in  the  salts  called  molybdates, 
of  which  lead  molybdate,  the  mineral  wulfenite,  is  the 
most  common. 

GOLD. 

Native  Gold,  Au. 

GOLD  is  the  most  highly  prized  of  the  metals,  valued 
because  it  serves  as  the  money  of  all  civilized  people,*  and 
because  of  its  use  for  ornaments,  as  watches,  rings,  etc. 

It  is  sometimes  found  in  isometric  crystals,  as  in  octa- 
hedrons, but  usually  in  plates  or  scales  or  wirelike  forms; 
also  in  larger  masses — sometimes  very  large — called  nug- 
gets (see  Fig.  163).  It  is  soft  (H.  =  2.5  to  3)  and  can  be 
cut  by  the  knife.  It  is  highly  malleable  and  ductile  and 
especially  remarkable  because  it  can  be  hammered  out  into 
very  thin  sheets;  the  skillful  gold-beater  can  make  the 
plates  so  thin  as  to  transmit  a  faint  greenish  light. 

Gold  is  very  heavy  and  when  pure  has  a  specific  gravity 
a  little  over  19.  The  luster  is  metallic  and  the  color  the 
familiar  gold-yellow,  but  varying  with  the  other  metals 

*  The  gold  coin  of  the  United  States  and  France  contains  gold 
and  copper  in  the  ratio  of  9  to  1;  that  of  England  in  the  ratio  of 
11  to  1. 


180  MINERALS,  AND   HOW  TO  STUDY  THEM. 

alloyed  with  it.  The  native  gold  practically  always  con- 
tains some  silver  and  often  a  good  deal,  and  then  it  has  a 
paler  color  and  lower  density;  with  sixteen  per  cent  of 
silver  the  specific  gravity  is  only  17.  The  gold  used  for 
watch-cases  and  for  ornaments,  on  the  other  hand,  is  often 
alloyed  with  copper  and  hence  has  a  reddish  color.  Gold 
is  not  attacked  by  the  ordinary  acids,  but  is  dissolved  in  a 
mixture  of  nitric  and  hydrochloric  acids  (called  aqua 
regia). 

Gold   occurs   mostly  in   veins   in    the  older  crystalline 

163. 


Figure  of  a  model  of  a  large  Australian  gold  nugget  weighing  2166  ounces  and 
valued  at  about  40,000  dollars. 

rocks,  especially  associated  with  quartz;  gold  quartz  is 
quartz— often  milky — which  either  shows  little  particles 
of  gold  scattered  through  it,  or  from  which  gold  can  be 
obtained — even  if  not  visible  to  the  eye — after  the  rock 
is  crushed  to  powder  and  then  washed  to  remove  the 
lighter  material.  A  large  part  of  the  gold  of  the  world 
has  been  obtained  from  the  sands  and  gravels  produced  by 
the  disintegration  of  gold-bearing  rocks.  These  gravels  in 
the  bed  of  a  stream  may  be  washed  by  the  miner  in  his 
pan;  or,  on  a  large  scale,  where  a  powerful  stream  of 


DESCRIPTION   OF   MINERAL   SPECIES.  181 

water  is  thrown  against  the  gravel  bank,  carrying  away 
the  lighter  rock  and  leaving  the  heavy  gold  particles  be- 
hind, usually  in  the  form  of  little  flattened  scales.  The 
finest  particles  preserved  are  called  "  gold-dust." 

The  chief  gold-producing  countries  at  the  present  time 
are  the  United  States,  especially  in  the  State  of  California, 
where  gold  was  discovered  in  1848;  in  Australia,  Eussia, 
South  Africa,  where  recent  discoveries  have  proved  to  be 
very  important.  Gold  is  also  produced  in  South  America, 
China,  British  India,  Canada;  to  a  limited  extent  in  Ger- 
many and  Austria-Hungary  and  some  other  countries. 

It  is  remarkable  that  almost  all  the  gold  of  the  world — 
and  an  amount  valued  at  about  $180,000,000  was  mined  in 
1894 — is  obtained  from  the  native  metal;  for  minerals 
containing  gold  are  very  rare.  The  only  ones  known,  be- 
sides the  auriferous  pyrite,  arseuopyrite,  etc.,  are  a  few 
compounds  with  tellurium  called  tellurides. 

The  best  known  of  these  gold  tellurides  is  SYLVANITE,  a 
silver-white  mineral  with  brilliant  metallic  luster,  soft 
(H.  =  1.5-2)  and  heavy  (G.  =  8.0).  It  was  long  since 
found  in  Transylvania  (whence  it  takes  its  name),  but  also 
occurs  in  Colorado.  Another  name  for  it  is  Graphic  Tel- 
lurium, because  of  the  curious  forms,  resembling  written 
characters,  that  the  crystals  sometimes  take  on  a  rock 
surface. 

PLATINUM. 

Native  Platinum,  Pt. 

PLATINUM  is  reckoned  among  the  nobler  metals  with 
gold,  and  like  it  is  not  attacked  by  any  of  the  single  acids. 


182  MINERALS,  AND    HOW   TO   STUDY   THEM. 

It  has  a  rather  dull  gray  color,  and  is  not  a  beautiful  metal, 
although  now  more  highly  valued  because  of  its  practical 
uses  than  any  of  the  metals  except  gold. 

It  is  rarely  found  in  isometric  crystals,  as  in  cubes,  more 
commonly  in  scales  or  in  larger  masses  (up  to  twenty 
pounds)  called  nuggets,  washed  out  of  the  gold  sand.  It 
has  a  hardness  of  4  to  4.5,  and  a  specific  gravity  varying 
from  14  to  19  according  to  the  amount  of  other  metals 
alloyed  with  it  chemically.  Pure  platinum,  as  obtained  in 
the  laboratory,  has  a  specific  gravity  of  21  to  22,  for  native 
platinum  is  not  the  pure  metal,  but  is  found  by  the  chemist 
to  contain  iron,  sometimes  in  large  amount  (nearly  20  per 
cent),  and  also  a  number  of  rare  metals,  as  palladium, 
rhodium,  and  others. 

Platinum  is  a  highly  useful  metal.  The  fact  that  it  is 
fused  with  great  difficulty  and  is  not  attacked  by  ordinary 
chemical  reagents  makes  it  very  valuable-  both  to  the 
chemist  in  the  laboratory  and  in  the  chemical  manufac- 
tories, where  crucibles  and  dishes  are  made  of  it.  It  is  also 
largely  used  by  dentists.  It  has  come  into  use  of  recent 
years  for  the  attachments  to  the  ends  of  the  carbon  wire  in 
the  incandescent  electric  lamp.  Only  a  very  minute 
quantity  is  required  in  each  case,  but  so  many  lamps  are 
called  for  that  the  demand  is  very  great,  and  as  only  a 
small  amount  is  mined — chiefly  in  the  Ural  Mountains  in 
Russia — the  price,  has  risen  much  higher  than  formerly. 
Platinum  has  been  used  to  a  small  extent  for  coins. 
Between  the  years  1828  and  1845  in  Russia  a  considerable 
amount  was  in  circulation,  but  the  coins  were  recalled  and 
the  experiment  has  not  been  repeated. 


DESCRIPTION   OF   MINERAL  SPECIES.  183 

Platinum,  like  gold,  does  not  readily  combine  with  other 
elements,  and  in  nature  the  only  compound  known  is  an 
arsenide  (PtAsJ,  called  SPERRYLITE;  this  is  found  in  very 
small  quantities  in  a  mine  near  Sudbury,  Ontario,  Canada. 
It  is  interesting  to  note  that  the  name  platinum  is  derived 
from  plata,  the  Spanish  word  for  silver,  since  it  was  re- 
garded in  South  America  at  the  time  of  its  discovery  (1735) 
as  an  impure  ore  of  that  metal. 

IRIDOSMINE  is  a  compound  of  the  rare  metals  iridium 
and  osmium  resembling  platinum,  but  of  a  whiter  color.  It 
is  found  under  similar  conditions  in  the  form  of  flattened 
scales  in  gold-washings.  It  is  very  hard,  and  on  this 
account  has  been  used  for  the  points  of  gold  pens. 

SILVER. 

SILVER  is  one  of  the  precious  metals,  useful  alike  as 
money,*  for  ornaments  of  many  kinds,  and  for  utensils. 
The  color  is  a  fine  silver-white  when  perfectly  fresh,  but 
unfortunately  it  is  very  easily  tarnished,  and  the  presence 
of  a  very  little  sulphur  or  sulphur  gases  in  the  atmosphere 
soon  turns  it  black. 

Native  Silver,  Ag. 

NATIVE  SILVER  is  not  an  uncommon  mineral,  although 
the  world's  supply  of  the  metal  comes  chiefly  from  its  ores. 
It  is  like  gold  in  its  occurrence,  sometimes,  though  rarely, 
in  distinct  isometric  crystals,  more  frequently  in  arbores- 

*  The  silver  coin  of  the  United  States  and  France  contains  silver 
and  copper  in  the  ratio  of  9  to  1 ;  that  of  Eugland  in  the  ratio  of  12$ 
tol. 


184  MINERALS,  AND    HOW   TO   STUDY   THEM. 

cent  or  branching  groups,  in  plates  and  scales  or  wirelike 
forms  (Fig,  164);  sometimes  in  fine  threads. 

Its  hardness  is  2.5  to  3;  it  is  highly  malle- 
able and  ductile,  and  is  the  best  known  con- 
ductor for  both  heat  and  electricity.  Its 
specific  gravity  is  10.6  when  pure,  but  higher 
when  alloyed  with  gold,  as  often  in  nature. 

Native  silver  occurs  rather  abundantly  in 
nature,  as  in  Mexico,  Arizona,  Norway,  also 
in  South  America  and  Australia. 

Silver  is  readily  dissolved  by  nitric  acid,, 
forming  silver  nitrate,  and  from  its  solution 
the  addition  of  any  compound  containing  chlorine,  as  hy- 
drochloric acid  or  sodium  chloride,  causes  part  to  separate 
as  a  white  curdy  deposit  of  silver  chloride.  This  is  a  very 
delicate  test  for  silver. 

Argentite,  or  Silver  Glance.     Silver  sulphide,  AgaS. 

Argentite  is  named  from  the  Latin  word  of  silver, 
argentnm.  It  is  a  very  valuable  though  not  very  common 
ore,  since  when  pure  it  contains  87  per  cent  of  metallic 
silver.  It  is  found  in  .cubic  or  octahedral  crystals,  often 
growing  together  in  branching  forms;  more  commonly  it 
occurs  simply  in  masses. 

The  hardness  is  about  2,  and  the  specific  gravity  7.3.  It 
is  readily  cut  with  the  knife,  almost  like  lead,  and  hence  is 
said  to  be  eminently  sectile,  also  flattening  to  some  extent 
under  the  hammer,  while  almost  all  other  sulphides  are 
brittle  and  break  at  once  with  a  blow  into  fragments.  The 
luster  is  metallic,  and  the  color  and  streak  grayish  black. 


DESCRIPTION   OF  MINERAL   SPECIES.  185 

The  formula  is  Ag,S,  or  silver  sulphide,  which  gives: 
Sulphur  12.9,  silver  87.1  =  100.  Heated  by  the  blowpipe 
flame  on  charcoal,  the  sulphur  is  easily  roasted  off  and  a 
little  silver  ball  left  behind,  which  can  be  tested  chemically 
by  dissolving  in  nitric  acid,  and  adding  a  drop  of  hydro- 
chloric acid,  as  before  mentioned. 

There  are  a  number  of  other  sulphur  compounds  of 
silver,  but  most  of  them  are  too  rare  to  be  mentioned  fully 
here.  The  most  interesting  of  these  are  the  two  beautiful 
minerals  called  red-silver  ore  or  ruby-silver,  that  is,  the  dark 
red-silver  ore,  PYRARGYRITE,  which  contains  sulphur, 
antimony,  and  silver,  am"  the  light  red-silver  ore,  PROUS- 
TT~E,  which  contains  sulphur,  arsenic,  and  silver. 

Both  these  minerals  crystallize  in  hexagonal  prisms  with 
rhombohedral  or  scalenohedral  faces,  and  they  resemble  each 
other  closely  in  their  characters,  as  hardness  2.5,  specific 
gravity  5.8  pyrargyrite  and  5.6  proustite.  The  color  of 
pyrargyrite  is  dark  red,  often  black,  with  nearly  metallic 
luster  on  the  surface,  while  proustite  is  bright  red.  Both 
have  a  red  streak. 

Heated  on  charcoal,  pyrargyrite  gives  off  dense  antimony 
fumes  (Sb203),  while  proustite  yields  arsenical  fumes 
(As203)  easily  recognized  by  their  garlic  odor.  Both  min- 
erals give  a  globule  of  silver  if  roasted  with  soda  on  charcoal 
(see  p.  144). 

Cerargyrite,  or  Horn-silver.     Silver  chloride,  AgCl. 

The  name  CERARGYRITE,  translated  into  English,  means 
horn-silver,  and  it  is  so  called  because  of  its  appearance 
and  the  ease  with  which  it  is  cut  by  a  knife. 


186  MINERALS,  AND   HOW   TO   STUDY   THEM. 

It  is  found  in  cubic  crystals  rarely,  more  commonly 
in  scales,  plates  or  masses.  The  hardness  is  1  to  1.5, 
and  the  specific  gravity  5.5.  It  is  remarkable  for  being 
perfectly  sectile,  cutting  with  a  knife  like  a  piece  of 
lead  or  wax.  The  luster  is  adamantine  and  the  color 
white  or  pale  gray  or  green;  it  is  transparent  to  trans- 
lucent. 

It  is  a  rather  rare  but  highly  valuable  silver  ore,  the  per- 
centage composition  being:  Chlorine  24.7,  silver  75.3  = 
100.  Eoasted  alone  on  charcoal  the  chlorine  is  easily 
driven  off  and  a  globule  of  silver  left  behind. 

MERCURY. 

MERCURY  is  a  remarkable  metal,  because  it  is  a  liquid  at 
all  ordinary  temperatures,  only  freezing,  or  becoming  solid, 
at  —  40°.  It  has  a  silver-white  color  and  brilliant  metallic 
luster,  and  is  so  mobile  that  from  early  times  it  has  been 
called  quicksilver.  Its  density  is  high,  13.6,  or  higher 
than  silver  (10.6)  and  lead  (11.4),  and  for  this  reason  and 
because  of  its  liquid  form  it  is  of  great  value  for  scientific 
purposes.  It  is  used  in  most  thermometers  and  barome- 
ters and  is  employed  in  many  experiments  in  the  physical 
and  chemical  laboratories.  It  also  has  the  property  of 
forming  a  pasty  mass  or  amalgam  with  some  of  the  other 
metals,  as  gold  and  silver  (also  copper,  zinc,  tin,  etc.,  but 
not  iron),  and  is  hence  of  great  value  in  separating  them 
from  the  rock  in  which  they  occur.  For  this  purpose  the 
rock  is  ground  into  powder,  the  greater  part  of  the  loose 
material  washed  off,  and  then  the  remainder  is  agitated 
with  mercury.  The  amalgam,  which  forms,  is  collected, 


DESCRIPTION   OF   MINERAL  SPECIES.  187 

and  by  heat  the  mercury  is  driven  off  to  be  collected  again 
in  cool  chambers  for  further  use,  and  the  gold  and  silver 
are  left  behind.  Ordinary  mirrors  are  made  of  glass 
backed  with  an  amalgam  of  mercury  and  tin.  The  sul- 
phide of  mercury  is  the  valuable  pigment  called  vermilion. 
Mercury  in  various  forms  is  also  used  in  medicine,  but  in 
minute  doses,  for  it  is  an  active  poison.  Corrosive  sub- 
limate is  a  chloride  of  mercury. 

NATIVE  MERCURY  is  a  rare  mineral  in  nature,  though 
occasionally  found  in  minute  globules  scattered  through 
the  rock;  the  common  ore  is  cinnabar. 

NATIVE  AMALGAM  is  a  rather  rare  mineral  containing 
mercury  and  silver,  but  in  very  varying  amounts. 

Cinnabar.     Mercury  sulphide,  HgS. 

CINNABAR,  the  sulphide  of  mercury,  and  sometimes 
called  natural  vermilion,  is  found  in  masses  of  a  fine  red 
color,  and  sometimes  also  in  small  rhombohedral  or  pris- 
matic crystals.  The  hardness  is  2  to  2.5,  and  the  specific 
gravity  is  about  8,  or  above  that  of  metallic  iron  (7.8). 
The  great  weight  of  a  specimen  cannot  escape  the  observer 
and  is  a  striking  character;  in  some  cases,  however,  if  the 
cinnabar  is  not  a  pure  solid  mass,  but  only  scattered 
through  a  light  clayey  gangue,  the  density  of  the  whole 
may  be  much  lower  than  8. 

The  luster  is  adamantine  and  the  color  bright  cochineal- 
red,  sometimes  becoming  dull  and  dark;  the  streak  is 
scarlet;  crystals  are  usually  perfectly  transparent. 

The  formula  for  mercury  sulphide,  HgS,  gives  the  per- 
centage composition:  Sulphur  13.8,  mercury  86.2  =  100. 


188  MINERALS,  AND   HOW  TO   STUDY   THEM. 

If  heated  on  charcoal,  a  piece  of  pure  cinnabar  is  volatilized 
entirely;  if  anything  is  left  behind,  it  is  only  the  gangue. 
In  the  closed  tube  it  is  also  sublimed  entire,  but  here  it 
collects  again  in  the  cold  part  of  the  tube  above  as  a  black 
ring  of  sulphide  of  mercury,  which  has  the  same  compo- 
sition as  the  original  mineral,  for  the  chemist  knows  both 
a  black  and  a  red  sulphide.  In  the  open  tube,  if  heated 
very  slowly,  so  as  to  avoid  forming  a  black  ring — in  other 
words,  so  as  to  give  the  sulphur  time  to  oxidize  (go  off  as 
S0a)— a  ring  of  metallic  mercury  is  formed  in  the  cold 
part  of  the  tube  (see  also  p.  148). 

Cinnabar  is  mined  at  Almaden  in  Spain,  Idria  in  Car- 
niola,  also  at  New  Almadeu  and  other  points  in  California, 
and  less  abundantly  elsewhere. 

COPPER. 

COPPER  is  one  of  the  most  useful  of  the  metals,  having 
been  employed  for  utensils  and  in  other  forms,  both  as  a 
metal  and  in  different  alloys,  since  very  early  times.  Of 
recent  years  its  use  has  been  increased  very  largely  be- 
cause of  its  good  conductivity  for  electricity.  It  thus 
forms  the  material  of  the  wires  of  the  dynamo  machines, 
those  by  which  the  electrical  current  is  carried  for  the  elec- 
tric light,  the  trolley,  etc.  Copper  is  also  extensively  used 
for  electroplating,  as  in  making  stereotype  plates.  It 
forms  further  a  large  number  of  useful  alloys,  of  which 
brass — an  alloy  of  copper  and  zinc  in  the  ratio  of  about 
2  :  1 — is  the  best  known.  In  the  various  kinds  of  bronze 
(bell-metal,  gun-metal,  antique  and  medal  bronze,  etc.) 
copper  is  also  the  prominent  metal,  alloyed  with  tin;  in 


DESCRIPTION   OF   MINERAL    SPECIES.  189 

aluminium  bronze  it  is  alloyed  with  aluminium;  in  german 
silver  it  is  alloyed  with  zinc  and  nickel. 

Copper  is  obtained  in  nature  in  the  native  state,  and  also 
from  a  variety  of  valuable  ores,  which  are  some  of  the  most 
interesting  and  beautiful  minerals. 

Native  Copper,  Cu. 

NATIVE  COPPER  is  found  sometimes  in  isometric  crys- 
tals, but  they  are  not  often  distinct,  and  the  common 
rorms  are  strings  or  wires  which  have  a  crystalline  form 
but  are  difficult  to  decipher  (see  Figs.  127,  128,  p.  61). 
It  is  also  in  grains,  plates,  and  masses,  sometimes  very 
large. 

The  hardness  is  2.5  to  3,  and  the  specific  gravity  8.8. 
The  luster  is  metallic  and  the  color  that  peculiar  reddish 
hue  called  copper-red.  It  is  highly  malleable  and  ductile, 
so  that  it  may  both  be  rolled  out  into  sheets  and  drawn 
into  fine  wires.  It  is  an  excellent  conductor  of  both  heat 
and  electricity.  Copper  is  easily  dissolved  by  nitric  acid, 
giving  a  blue  solution,  and  ammonia  in  excess  (enough  to 
dissolve  the  precipitate  first  formed)  turns  it  a  deep  azure- 
blae. 

The  most  celebrated  locality  for  native  copper  is  in  the 
upper  peninsula  of  Michigan  on  the  shores  of  Lake  Su- 
perior, where  it  has  been  mined  for  many  years.  The 
total  production  has  been  very  large.  Beautiful  crys- 
tallized specimens  have  been  found  here  where  it  is  asso- 
ciated with  calcite,  datolite,  and  a  number  of  the  zeolites. 
Sometimes  it  is  inclosed  in  the  crystals,  as  of  calcite,  so 
that  they  are  colored  bright  red  from  the  internal  reflec- 


190  MINERALS,  AND   HOW  TO  STUDY  THEM. 

tions.  Great  masses  of  native  copper  hare  also  been 
found;  one  of  them  weighed  420  tons.  Native  copper  is 
further  found  in  Arizona,  in  Siberia,  South  America,  and 
Australia. 

Chalcocite,  or  Copper  Glance.     Cuprous  sulphide,  Cu.S. 

CHALCOCITE  is  one  of  the  most  valuable  ores  of  copper, 
for  when  pure  it  contains  about  80  per  cent  of  the  metal. 
It  is  found  in  orthorhombic  prisms  or  pyramids,  occasion- 
ally having  a  hexagonal  aspect;  more  commonly  in  mas- 
sive forms  of  a  nearly  black  or  bluish-black  color.  When 
fresh  it  has  a  brilliant  metallic  luster,  which  it  loses  easily, 
becoming  a  little  dull  and  tarnished  on  the  surface. 

The  hardness  is  2.5  to  3,  and  the  specific  gravity  about 
5.6.  It  is  brittle  when  struck  with  the  hammer,  but  can 
be  cut  a  little  with  the  knife. 

The  formula  for  chalcocite  (cuprous  sulphide),  CuaS, 
gives  the  composition :  Sulphur  20.2,  copper  79.8  =  100. 
On  charcoal  it  is  easily  reduced  by  the  blowpipe  flame 
alone  to  metallic  copper.  Fine  specimens  come  from 
Cornwall  in  England  (often  called  redruthite) ;  it  was 
also  formerly  obtained  at  Bristol,  Conn. 

Bornite,  or  Erubescite.     Sulphide  of  Copper  and  Iron, 
Cu3FeS8. 

BORNITE  was  named  after  the  Austrian  mineralogist 
von  Born,  but  it  has  a  variety  of  other  names— purple  cop- 
per ore,  variegated  copper  ore,  peacock  copper,  erubescite— 
all  of  which  suggest  a  character  by  which  it  is  easily 
recognized:  the  bright  iridescent  tarnish  of  the  surface. 


DESCRIPTION   OF   MINERAL   SPECIES.  191 

A  fresh  fracture  gives  a  color  of  a  peculiar  reddish  bronze 
and  a  bright  metallic  luster,  which  has  led  the  Cornish 
miners,  a  little  fancifully,  to  call  it  horse-flesh  ore.  This 
fresh  surface  soon  becomes  slightly  colored  even  after  a 
day  or  two,  and  gradually  the  color  changes  and  becomes 
more  variegated,  until  it  is  indeed  a  peacock-copper  ore. 
This  character,  with  the  peculiar  color  of  the  fresh  frac- 
ture, makes  it  always  easy  to  recognize.  It  is  sometimes 
found  in  cubic  crystals,  but  usually  it  is  simply  massive  as 
imbedded  particles  or  larger  pieces.  The  hardness  is  3, 
and  the  specific  gravity  about  5. 

Bornite  contains  both  copper  and  iron,  but  not  always 
in  the  same  proportions;  the  formula  Cu3FeSs  gives: 
Sulphur  28.1,  copper  55.5,  iron  16.4  =  100.  When  heated 
in  the  open  tube  it  gives  off  fumes  of  sulphur  dioxide, 
which  are  recognized  by  the  odor  and  their  effect  in  red- 
dening litmus-paper.  On  charcoal  it  fuses  to  a  brittle 
magnetic  globule;  after  roasting  it  reacts  with  borax  for 
iron  and  copper.  It  dissolves  in  nitric  acid  with  separation 
of  sulphur,  giving  a  blue  solution. 

Chalcopyrite,  or  Copper  Pyrites.    Sulphide  of  Copper  and 
Iron,  CuFeS,. 

CHALCOPYRITE,  or  Copper  Pyrites,  is  the  beautiful  deep 
brass-yellow  copper  mineral,  often  called  yellow  copper 
ore.  The  color  is  so  golden  that  it  is  not  infrequently 
mistaken  for  gold,  especially  when  scattered  in  small  par- 
ticles through  a  mass  of  quartz;  but, as  we  shall  see,  it  can 
be  easily  distinguished,  though  the  name  "fool's  gold/' 


192  MINERALS,  AND  HOW  TO   STUDY  THEM. 

which  it  shares  with  the  less  golden  iron  pyrites,  is  still 
not  inappropriate. 

It  is  generally  found  massive,  sometimes  in  large  speci- 
mens, sometimes  only  in  specks  in  the  inclosing  rock,  but 
165  it   is   also   found  in  crystals  which   com- 

monly are  either  like  octahedrons  (though 
belonging  to  the  tetragonal  system),  or  in 
wedge-shaped  forms  called  sphenoids  (Fig. 
165). 

The  hardness  of  chalcopyrite  is  3.5  to  4, 
so  that,  unlike  pyrite,  it  can  be  easily  scratched  with  a 
knife.  It  is  brittle,  and  its  specific  gravity  is  a  little  over 
4.  The  luster  is  brilliant  metallic,  and  the  color,  as  we 
have  seen,  deep  brass-yellow;  the  streak  is  greenish  black. 
It  is  often  tarnished  on  the  surface,  sometimes  so  as  to 
deepen  the  color,  sometimes  variegated  so  as  to  rival  bor- 
nite,  with  which  it  might  then  be  confounded,  only  that 
the  breaking  off  of  a  scale  so  as  to  show  the  color  on  the 
fresh  fracture  serves  to  distinguish  them  at  once. 

It  is  a  sulphide  of  both  copper  and  iron,  and  the  formula 
CuFeS,  gives:  Sulphur  35.0,  copper  34.5,  iron  30.5  =  100. 
Heated  on  charcoal,  a  fragment  fuses  to  a  black  ball  which 
is  strongly  magnetic,  and  this  roasted  with  soda  gives 
metallic  copper.  A  fragment  in  nitric  acid  dissolves,  giv- 
ing a  blue  solution  which  turns  azure-blue  when  ammonia 
is  added  in  excess. 

Chalcopyrite  can  be  easily  distinguished  from  pyrite 
(iron  pyrites)  because  of  its  inferior  hardness,  as  noted  be- 
fore; its  color,  too,  is  deeper.  It  is  distinguished  from 
gold  by  its  being  brittle,  breaking  into  fragments  under 


DESCRIPTION   OF   MINERAL   SPECIES.  193 

the  point  of  the  knife,  while  the  gold  is  cut;  a  particle  of 
gold,  too,  is  not  attacked  by  nitric  acid,  while  the  chalco- 
pyrite  is  easily  dissolved  with  the  separation  of  particles  of 
sulphur.  It  is  a  very  common  mineral,  often  occurring  in 
/eins  of  quartz  with  galena  and  pyrite,  though  sometimes 
cnly  as  minute  specks.  When  present  in  large  masses,  as 
iu  Montana,  it  is  one  of  the  valuable  ores  of  copper. 

Tetrahedrite,  or  Gray  Copper.     Sulphide    of    Antimony 
and  Copper,  4CuaS.SbaSs. 

TETRAHEDRITE  is   so   named  because  the   crystals  are 
Commonly  tetrahedral  in  habit  (Figs.  166,  167),  and  often 
highly  modified.     Good  crystals,  as  with  many  of  these 
166.  '       167. 


metallic  minerals,  are  rare  and  the  mineralogist  has  often 
to  content  himself  with  massive  pieces.  These  he  recog- 
nizes by  the  brilliant  metallic  luster  and  dark  grayish-black 
color  and  streak.  The  hardness  is  3-4.5,  so  that  it  is  easily 
distinguished  from  magnetite,  which  is  too  hard  to  be 
scratched  by  the  knife;  the  specific  gravity  varies  from  4.4 
to  5.1. 

The  ordinary  tetrahedrite  contains  sulphur,  antimony, 
and  copper,  but  there  are  a  great  many  varieties,  some  of 
which  contain  arsenic  in  place  of  part  of  the  antimony,  and 
others  silver  or  mercury  in  place  of  part  of  the  copper. 


194  MINERALS,  AND  HOW  TO   STUDY   THEM. 

The  typical  composition  is  given  by  the  formula  CugSb.,S, 
or  4Cu,S.Sb,Ss;  this  requires:  Sulphur  23.1, antimony  24.8, 
copper  52.1.  There  is  also  a  related  mineral,  containing 
sulphur,  arsenic,  and  copper,  which  is  called  tennantite. 

In  the  closed  tube  tetrahedrite  gives  a  dark  red  subli- 
mate of  antimony  oxysulphide;  in  the  open  tube  sulphurous 
fumes  and  a  white  coating  of  antimony  trioxide.  If  arsenic 
is  present,  it  is  detected  by  the  odor  when  the  mineral  is 
heated  on  charcoal;  with  soda  it  yields  a  globule  of 
metallic  copper.  Cornwall,  Bohemia,  Hungary,  also  Col- 
orado, afford  fine  specimens. 

Cuprite,  or  Eed  Copper  Ore.     Cuprous  oxide,  Cu,0. 

CUPEITE  is  called  red  copper  because   of  the  fine  red 
color  which  the  clear  crystals  show,  and  because  of  the  red 
color  of  the  streak.     The  crystals  (Figs.  168-170)  are  often 
168.  169.  170. 


cubes  or  octahedrons  or  combinations  of  them  and  other 
forms;  sometimes  they  are  highly  modified  (see  Fig.  29,  p. 
28).  In  one  kind  the  cubes  are  spun  out  into  long  threads, 
forming  a  matted  mass  of  bright  red  hairs  which  look  very 
pretty  in  the  cavities  of  the  rock;  examined  closely  with  a 
glass,  it  is  often  seen  that  these  threads  cross  each  other 
at  right  angles  as  if  trying  to  build  up  skeleton  cubes,  the 
threads  taking  the  direction  of  the  cubic  edges.  Commor 


DESCRIPTION    OF   MINERAL   SPECIES.  195 

cuprite  is  a  massive  mineral,  and  it  is  in  its  cavities  that 
the  crystals  are  usually  found. 

The  hardness  of  cuprite  is  3.5  to  4,  and  the  specific 
gravity  about  6.  The  luster  is  adamantine,  but  on  some 
dark  surfaces  may  look  almost  metallic;  again  it  is  dull 
and  earthy.  The  color,  as  remarked  above,  is  bright  cochi- 
neal-red in  the  clear  transparent  crystals,  but  the  surface  is 
often  darkened  and  may  appear  nearly  black.  The  streak 
is  always  brownish  red. 

The  composition  of  cuprous  oxide,  Ci^O,  gives:  Oxygen 
11.2,  copper  88.8  =  100.  A  fragment  on  charcoal  is  easily 
robbed  of  its  oxygen  and  reduced  to  metallic  copper. 
Cuprite  is  not  only  a  beautiful  mineral,  but  also  a  valuable 
ore  of  copper,  occurring  usually  with  malachite  and  othef 
ores,  as  in  Arizona,  Cornwall  in  England,  in  Australia, 
etc. 

Malachite.     Green  Carbonate  of  Copper,  CuCO,.  Cu(OH),. 

MALACHITE,  the  carbonate  of  copper,  is  a  bright  green 
mineral,  often  found  with  native  copper,  cuprite,  and  othef 
copper  ores  because  of  the  readiness  with  which  they  are 
converted  into  the  carbonate  by  the  action  of  the  carbon 
dioxide  present  in  the  air  or  dissolved  in  the  water. 

It  may  be  found  in  acicular  crystals  (monoclinic),  but  onlj 
rarely,  and  the  common  forms  have  a  rounded  or  mammil- 
lary  surface  and  a  concentric  fibrous  structure  (see  Figs. 
135,  136,  p.  68).  When  close  and  compact  it  can  be  cut 
and  polished  and  thus  form  a  handsome  ornamental  stone 
The  malachite  of  Siberia  is  used  in  this  way,  table-tops 
vases,  and  columns  having  often  been  veneered  with  it. 


196  MINERALS,  AND   HOW  TO   STUDY  THEM. 

The  hardness  is  3.5  to  4,  and  the  specific  gravity  about  4. 
The  color  is  a  bright  green;  the  streak  a  little  paler;  it  is 
transparent  only  in  minute  crystals. 

The  formula  of  malachite  is  CuC08.Cu(OH),  or  2CuO, 
CO..H.O,  which  gives:  Carbon  dioxide  (CO,)  19.9,  cuprio 
oxide  (CuO)  71.9,  water  (HaO)  8.2  =  100.  A  fragment 
heated  in  the  forceps  gives  a  green  flame  characteristic  of 
copper,  and  in  the  borax  bead  the  reactions  described  on  pp. 
136,  137.  It  yields  a  good  deal  of  water  in  the  closed  tube, 
and  in  nitric  acid  dissolves  with  the  effervescence  of  carbon 
dioxide.  Malachite  is  found  in  fine  specimens  at  many 
localities,  as  in  the  Siberian  mines,  in  Cornwall,  Australia, 
and  Arizona. 

Azurite,  Blue  Carbonate  of  Copper.    2CuC03.Cu(OH)3. 

AZURITE,  or  the  Blue  Carbonate  of  Copper,  is  not  so 
common  as  malachite,  but  it  is  also  a  beautiful  mineral,  and 
when  in  large  transparent  crystals  of  a  fine  deep  blue  it 
forms  one  of  the  most  attractive  specimens  in  a  cabinet. 
The  crystals  are  oblique  rhombic  prisms.  The  hardness  is 
3.5  to  4,  and  the  specific  gravity  is  3.8.  The  luster  is 
vitreous,  the  color  azure-blue,  and  the  streak  somewhat 
lighter. 

The  composition  is  expressed  by  the  formula  2CuC03. 
Cu(OH)a  or  SCuO.gCOj.H.O;  this  gives:  Carbon  dioxide 
(C0a)  25.6,  cupric  oxide  (CuO)  69.2,  water  (H30)  5.2  = 
100.  It  hence  differs  from  malachite  in  containing  less 
water;  it  is  not  uncommon  to  find  crystals  which  are  blue 
on  the  outside  but  have  changed  within  to  a  fibrous  mass 
of  green  malachite.  The  most  famous  localities  are  those 


DESCRIPTION   OF   MINERAL   SFtiCIES.  197 

of  Chessy,  near  Lyons  in  France,  and  the  Copper  Queen 
mines  and  elsewhere  in  Arizona. 

DIOPTASE,  or  Emerald  Copper,  the  silicate  of  copper,  has 
a  beautiful  emerald-green  color.  It  is  a  rare  mineral,  only 
known  to  occur  at  a  few  localities,  one  of  which  is  in  Asia, 
another  in  Arizona,  another  in  the  French  Congo  re- 
gion in  Africa.  The  crystals  are  commonly  hexagonal 
prisms  with  rhombohedral  faces  on  the  ends.  The  formula 
is  H.O.CuO.SiO,,  which  gives  :  Silica  38.2,  cupric  oxide 
50.4,  water  11.4  =  100. 

CHRYSOCOLLA  is  another  silicate  of  copper  of  a  bluish- 
green  or  sky-blue  color.  It  occurs  in  massive  forms  some- 
times earthy,  also  looking  a  little  like  malachite.  The  hard- 
ness is  2  to  4;  the  specific  gravity  2.2.  It  contains  a  good 
deal  of  water,  which  it  gives  off  in  the  closed  tube.  The 
formula  is  CuSi03.2HaO.  It  is  a  not  uncommon  product 
of  the  alteration  of  other  copper  minerals. 

There  are  many  more  copper  minerals,  most  of  them  too 
rare  to  be  described  here.  They  include  a  number  of 
hydrous  sulphates,  of  which  the  most  important  is  CHAL- 
CANTHITE  or  blue  vitriol,  a  common  substance  at  the 
druggist's  and  often  to  be  seen  in  clusters  of  large  crystals, 
but  rare  in  nature.  The  sulphate,  BROCHANTITE,  may  also 
be  mentioned.  There  are  further  several  arsenates  of 
copper,  including  OLIVENITE  ;  several  phosphates,  includ- 
ing LIBETHENITE;  several  chlorides,  as  ATACAMITE;  and  so 
on. 

LEAD. 

LEAD  is  one  of  the  most  important  of  the  metals,  used  for 
many  purposes  familiar  to  all,  as  for  pipes,  to  convey  water, 


198 


MINERALS,  AND    HOW   TO   STUDY   THEM. 


for  shot  and  rifle-balls,  etc.  It  has  a  dull  blue-gray  color. 
It  is  very  soft  and  malleable,  and  fuses  readily  at  a  com- 
paratively low  temperature  (see  p.  132.)  It  is  often  alloyed 
with  other  metals;  thus  with  tin  in  common  solder  and 
pewter;  with  antimony  in  type-metal ;  with  arsenic  in 
small  amount  for  making  shot.  White  lead  (the  carbonate) 
:s  largely  used  in  making  paint,  also  the  oxide,  red  lead. 

NATIVE  LEAD  is  a  very  rare  mineral,  though  occasion- 
ally found  in  small  amount,  particularly  in  Sweden.  The 
supply  of  the  metal,  which  is  used  so  largely  in  the  arts,  is 
obtained  from  its  ores,  especially  the  culphide,  galena. 
Other  important  lead  minerals  are  the  phosphate,  pyro- 
morphite;  the  sulphate,  anglesite;  the  carbonate,  cerussite. 

Galena.     Lead  sulphide,  PbS. 

GALENA  crystallizes  in  the  isometric  system,  and  occurs 

commonly  in  cubes ;  it  is  also  found  in  octahedrons  and 

very  frequently  in  combinations  of  these  two  forms  as,  too, 

of  other  forms  of  this  system  (Figs.  171-173,  also  Fig.  20, 

171.  172.  173. 


p.  25).  It  has  very  perfect  cubic  cleavage,  and  a  mass  often 
breaks  up  into  a  multitude  of  little  rectangular  blocks  (see 
Fig.  143,  p.  71).  This  cubic  cleavage  is  readily  seen  in  the 
common  coarse-granular  kinds,  and  is  revealed  also  by  the 
spangling  of  the  surface  in  those  which  are  fine-granular. 


DESCRIPTION   OF   MINERAL   SPECIES.  M* 

The  hardness  is  2.5,  and  the  specific  gravity  is  7.5  or  nearly 
as  high  as  metallic  iron,  for  lead  being  a  metal  of  high 
density  (G.  =  11.4),  all  its  compounds  have  the  same  prop- 
erty. 

The  luster  is  metallic  and  usually  very  brilliant;  the 
color  is  a  bluish  lead-gray,  but  the  exposed  surface  of  a 
specimen  is  often  somewhat  dull  from  tarnish. 

Galena  is  lead  sulphide,  PbS,  which  gives  the  percentage 
composition:  Sulphur  13.4, lead  86.6  =  100.  On  charcoal  a 
fragment  fuses  easily,  yielding  finally  a  globule  of  metallic 
lead  and  a  yellow  coating  of  lead  oxide  near  it  and  a  white 
coating  at  a  distance  from  it  (see  p.  144).  With  soda  on 
charcoal  metallic  lead  is  readily  obtained. 

Galena  is  the  most  important  ore  of  lead  and  one  of  the 
commonest  of  minerals,  occurring  in  large  deposits  in  many 
mining  regions;  for  example,  in  Missouri,  Illinois,  Iowa, 
and  Wisconsin;  also  in  Colorado  and  abroad  in  Derbyshire, 
England;  Freiberg,  the  Harz,  and  soon.  It  also  occurs, 
but  less  abundantly,  with  the  ores  of  other  metals.  Spha^ 
lerite,  calamine,  smithsonite,  also  pyrite  and  chalcopyrite 
are  common  accompanying  metallic  minerals;  quartz,  cal- 
cite,  barite,  also  fluorite,  are  common  non-metallic  minerals 
associated  with  it  and  then  called  the  gangue.  As  resulting 
from  its  own  decomposition,  lead  carbonate  (cerussite)  and 
also  lead  sulphate  (anglesite)  are  often  found  with  galena; 
less  often  pyromorphite  and  other  lead  minerals.  Much 
galena  carries  a  small  amount  of  silver,  and  when  this  is 
sufficient  in  quantity  to  justify  its  being  worked  for  the 
precious  metal,  it  is  regarded  as  a  silver  ore  and  called 
argentiferous  galena. 


200  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Galena  is  used  not  only  as  a  source  of  lead  or  sometimes 
of  silver,  but  also  for  glazing  common  stoneware;  it  is 
hence  called  potter's  ore. 

JAMESONITE  is  a  rare  sulphide  of  lead  and  antimony 
(2PbS.SbaSs)  occurring  in  acicular  crystals,  also  in  fibrous  or 
compact  masses.  Hardness  2  to  3;  specific  gravity  5.5  to 
6.  The  luster  is  metallic  and  the  color  is  steel-gray  to  dark 
lead -gray  ;  it  resembles  stibnite  both  in  form  and  color. 

BOURNONITE  is  another  Bather  rare  sulphide  of  lead 
and  antimony  containing  also  copper  (3(Pb,Cu)S.Sb2Sg). 
It  occurs  in  short  prismatic  or  tabular  crystals,  often 
grouped  in  wheel-shaped  forms  ;  also  massive  and  compact. 
Hardness  2.5  to  3  ;  specific  gravity  5.7  to  5.9  ;  luster 
metallic ;  color  dark  steel-gray,  inclining  to  iron-black. 

Pyromorphite.     Lead  phosphate,  3Pb3P308.PbCl4. 

PYROMORPHITE  is  found  in  small  hexagonal  prisms  which 
are  sometimes  cavernous  in  form,  also  often  rounded  into 
barrel-shaped  forms,  or  even  nearly  spheri- 
cal. The  crystals  are  frequently  clustered 
together  in  a  curious  way,  branching  out 
from  a  slender  stem,  as  shown  in  Fig.  174. 
It  also  occurs  as  a  thin  crust  or  coating, 
which  may  be  drusy  on  the  surface,  or  simply 
globular  or  mammillary. 

Pyromorphite  has  a  hardness  of  3.5  to  4, 
and,  like  all  compounds  of  lead,  a  high  specific 
gravity,  viz.,  6.5  to  7.  The  luster  is  resinous  and  the  color 
is  commonly  green,  varying  from  grass-green  to  both  darker 
and  lighter  shades;  it  is  also  sometimes  pale  brown.  The 


DESCRIPTION   OF  MINERAL  SPECIES.  201 

streak  is  not  far  from  white  even  in  the  deep  green 
varieties. 

It  consists  essentially  of  phosphate  of  lead,  Pb,(P04)s, 
but  contains  also  some  chlorine.  Hence  when  heated  in 
the  tube  a  little  lead  chloride  is  driven  off  and  forms  a 
white  coating  above.  The  same  white  coating  is  also  de- 
posited on  charcoal  at  a  distance  from  the  fragment  which 
is  being  heated ;  more  conspicuous  than  this  is  the  yellow 
coating  of  lead  oxide  (PbO)  which  is  formed  just  about  the 
fused  fragment.  Also  if  the  fragment,  after  it  is  com- 
pletely fused,  is  examined,  it  will  be  seen  that  it  is  nearly 
spherical,  has  a  brilliant  luster,  and  sparkles  on  the  surface 
from  the  reflection  of  light  from  a  multitude  of  crystalline 
facets;  it  is  this  that  gives  the  name  to  the  mineral 
from  the  Greek  words  meaning  fire  and  form  (nvp  and 
fiiop<fit'/).  The  fused  globule,  if  heated  further  on  charcoal 
with  the  addition  of  some  sodium  carbonate,  yields  globules 
of  metallic  lead. 

MIMETITE  and  VANADINITE  are  two  ores  of  lead  which 
are  closely  related  in  form  and  composition  to  pyromor- 
phite. 

Mimetite  consists  essentially  of  lead  arsenate,  and  vana- 
dinite  of  lead  vanadate,  and  each,  like  pyromorphite,  also 
contains  a  little  chlorine.  Certain  intermediate  varieties 
contain  both  arsenic  and  vanadium. 

Mimetite  is  usually  yellow  in  color  and  has  a  resinous 
luster;  it  sometimes  closely  resembles  pyromorphite  in 
form  (hence  the  name  from  /IZ/O/TT/S,  imitator),  but  dis- 
tinct crystals  are  more  rare  and  rounded  indistinct  forms 
the  rule.  The  hardness  is  3.5,  and  the  specific  gravity  7 


202  MINERALS,  AND   HOW   TO  STUDY  THEM. 

to  7.25.  It  is  easily  recognized  by  yielding  arsenical  fnmea 
on  charcoal  with  their  peculiar  odor,  while  the  reactions  for 
lead  are  the  same  as  for  pyromorphite. 

Vanadinite  is  often  of  a  beautiful  deep  red  color,  and 
when  the  crystals  are  clear  and  sharp  it  is  one  of  the  most 
beautiful  of  minerals,  especially  in  the  forms  found  of  late 
years  in  some  of  the  mining  regions  of  Arizona;  less 
brilliant  yellow  and  light  brown  varieties  also 
occur.  The  crystals  are  hexagonal  prisms, 
often  terminated  by  several  hexagonal  pyra- 
mids; cavernous  forms  also  occur  as  with 
pyromorphite  (Fig.  175).  The  hardness  is 
about  3,  the  specific  gravity  6.7  to  7.  The  luster  is  resin- 
ous. 

The  reactions  for  lead  on  charcoal  are  like  those  of  pyro- 
morphite; the  vanadium  is  recognized  by  the  yellow  and 
emerald-green  colors  which  it  gives  to  the  salt  of  phosphorus 
bead,  the  former  in  the  oxidizing,  the  latter  in  the  reduc- 
ing flame  (see  p.  139). 

It  may  be  interesting  to  add  that  the  rare  element  vana- 
dium, present  in  vanadinite  and  some  of  the  vanadates  (as 
descloizite,  a  vanadate  of  lead,  zinc,  and  copper),  has  found 
a  use  in  the  arts  in  calico-printing;  it  is  also  used  for 
vanadium  black,  in  making  ink,  and  to  fix  the  colors  in 
the  manufacture  of  silk. 

Somewhat  resembling  vanadinite  in  bright  color  are  two 
rather  rare -lead  minerals,  Crocoite  or  lead  chromate, 
PbCrO,,  and  Wulfenite  or  lead  molybdate,  PbMo04. 

CROCOITE  occurs  in  oblique  prismatic  crystals  (mono- 
clinic)  of  a  fine  orange-red  color  and  giving  a  deep  yellow 


DESCRIPTION    OF   MINERAL   SPECIES.  203 

streak.  The  hardness  is  2.5  to  3,  and  the  specific  gravity 
about  6.  The  luster  is  adamantine  to  vitreous.  It  is 
recognized  by  the  lead  coating  which  it  gives  on  charcoal, 
and  by  the  reactions  for  chromium,  which  yields  a  green 
bead  with  borax  in  both  the  oxidizing  and  reducing 
flames  (see  p.  138). 

WULFENITE  is  not  so  rare  as  crocoite,  and  is  one  of  the 
many  fine  minerals  which  the  western  mining  States  have 
afforded  in  great  variety  and  beauty,  175. 

where  it  often  occurs  with  vanadin- 
ite.  It  is  found  in  square  crystals 
(Fig.  176)  or  tables  frequently  as  thin  as  a  knife-edge  and 
perfectly  clear;  also  in  square  pyramids,  sometimes  low 
and  less  often  acute.  The  color  is  a  bright  orange-yellow 
to  reddish  yellow,  or  again  brown  or  green;  the  luster  is 
resinous  or  adamantine.  The  hardness  is  about  3,  and  the 
specific  gravity  6.7  to  7.  The  square  form  and  bright 
color  make  it  usually  easy  to  recognize  it. 

With  the  blowpipe  on  charcoal  it  gives  a  lead  coating, 
and  with  soda  metallic  lead.  With  salt  of  phosphorus  it 
gives  the  reactions  for  molybdenum  (p.  139). 

Cerussite,  or  White-lead  Ore.     Lead  carbonate,  PbC08. 

CERUSSITE,  the  carbonate  of  lead,  is  the  commonest  ore 
of  lead  next  to  galena,  and  with  this  it  commonly  occurs 
for  the  simple  reason  that  in  nature's  laboratory,  where 
carbon  dioxide  (CO,)  is  often  present,  the  sulphide  of 
Jead  (PbS)  is  often  changed  into  the  carbonate  (PbCOs). 
This  is  a  good  illustration  of  what  mineralogists  call  the 
association  of  minerals,  often  a  very  important  guide  as  to 


204 


MINERALS,  AND   HOW   TO  STUDY   THEM. 


their  nature.  It  is  in  the  same  way,  as  has  been  stated^ 
that  the  carbonate  of  copper  (malachite)  often  occurs  with 
the  sulphides,  the  oxides,  etc. 

The  clear  and  colorless,  or  perhaps  white  or  slightly  yel- 
low, crystals  of  cerussite  do  not  perhaps  at  first  suggest  to 
the  eye  that  it  is  a  mineral  containing  the  metal  lead.  A 


177. 


179. 


more  careful  examination,  however,  shows  the  adamantine 
luster,  and  this,  as  has  been  stated  (p.  89),  belongs  to  crys- 
tals which  refract  the  light  largely,  in  general  either  be- 
cause they  are  very  hard  (as  the  diamond),  or  because  they 
contain  heavy  atoms,  as  those  of  lead.  It  is  a  good  thing  to 
remember  that  all  the  compounds  of  lead  have  an  adaman 
tine  or  a  resinous  luster,  and  the  same  is  true  of  the  kind 
of  glass  containing  much  lead  called  paste,  out  of  which 
imitation  jewels  are  made. 

The  crystals  of  cerussite  are  sometimes  thin  plates  (Fig. 
177),  with  sides  covered  with  fine  horizontal  lines;  but 
rhombic  prisms  and  pyramids  also  occur;  Fig.  178  resem- 
bles a  hexagonal  pyramid  in  form  and  angle.  This  is  one 
of  the  species  which  is  frequently  in  twins,  and  six -rayed 
starlike  forms,  in  which  the  branches  cross  at  angles  of 
60°  and  120°,  are  common  (Fig.  179,  also  Fig.  120,  p.  58). 

The  hardness  is  only  3  to  3.5,  and  the  specific  gravity  is 


DESCRIPTION   OF   MINERAL   SPECIES.  205 

high,  about  6.5,  as  must  be  true  of  a  compound  of  lead ; 
this  last  character  is  one  that  should  be  noticed  first  if  a 
mass  approximately  compact  is  in  the  hand.  Sometimes, 
however,  the  mass  of  crystals  is  so  open  and  porous  that 
the  effect  of  high  density  is  lost. 

The  composition,  lead  carbonate,  PbCOs,  gives:  Carbon 
dioxide  (C03)  16.5,  lead  oxide  (PbO)  83.5  =  100.  Heated 
on  charcoal  carefully  (for  it  decrepitates  at  first)  a  fragment 
is  easily  fused  and  soon  yields  a  globule  of  metallic  lead, 
while  about  it  the  familiar  lead  coating  is  formed.  Placed 
in  a  test-tube  with  a  little  nitric  acid,  the  carbon  dioxide 
(CO,)  is  given  off  with  effervescence.  This  species  is  com- 
mon in  Colorado  and  other  mining  regions;  it  is  valuable 
as  an  ore  of  lead  and  often  of  silver.  It  is  the  same  as  the 
artificial  white  lead  used  for  paint. 

Anglesite.     Lead  sulphate,  PbS04. 

ANGLESITE  resembles  cerussite,  and  like  it  is  frequently 
found  in  cavities  in  the  sulphide,  galena  (PbS),  from  Avhich 
it  is  formed  by  oxidation.  It  is  often  in  crystals,  which 
are  either  rhombic  prisms  or  pyramids,  and  sometimes  very 
highly  modified ;  the  crystals  resemble  barite  in  form  and 
angles  (see  p.  262).  Cleavage  exists  parallel  to  the  prism 
and  the  base,  but  it  is  interrupted.  It  is  also  found  in 
closely  compact  forms  which  are  not  so  easy  to  recognize, 
but  whose  high  specific  gravity  is  suggestive. 

The  hardness  of  anglesite  is  2.75  to  3,  and  the  specific 
gravity  6.2  to  6.4.  The  luster  is  adamantine  in  most 
cases,  especially  on  a  fracture  surface,  sometimes  varying 


206  MINERALS,  AND   HOW   TO    STUDY   THEM. 

to  resinous.     The  crystals  are  usually  clear  and  colorless, 
but  the  masses  may  be  brown  and  nearly  opaque. 

The  formula  for  lead  sulphate,  PbS04,  gives:  Sulphur 
trioxide  (SO,)  26.4,  lead  protoxide  (PbO)  73.6  =  100.  On 
charcoal  before  the  blowpipe  it  decrepitates  and  fuses 
readily  to  a  clear  bead,  which  becomes  milk-white  on  cool- 
ing; in  the  reducing  flame  it  yields  metallic  lead.  With 
soda  on  charcoal  lead  is  easily  obtained,  and  the  soda  reacts 
for  sulphur  as  explained  on  p.  146.  It  dissolves  with  dif- 
ficulty in  nitric  acid,  and  does  not  effervesce  as  does  cerus- 
site,  and  hence  the  two  are  easily  distinguished  in  this  way. 
Beautiful  crystals  come  from  Anglesea  (whence  the  name), 
from  Sardinia,  from  Phoenixville,  Penn.  It  occurs  in  large 
quantities  in  Mexico  and  Australia. 

TIN, 

TIN,  one  of  the  most  important  of  metals  for  a  great 
variety  of  technical  purposes,  occurs,  if  at  all,  only  very 
rarely  in  nature  in  the  native  metallic  state.  The  supply 
is  obtained  almost  solely  from  a  single  ore,  the  mineral 
cassiterite,  or  tin-stone.  There  is  also  a  rare  sulphide 
of  tin,  copper,  and  iron,  called  STANNITE.  The  use  of  tin 
that  first  suggests  itself  is  for  tin  plate,  so  largely  employed 
for  vessels,  roofing,  etc;  this  is  simply  sheet  iron  coated 
with  metallic  tin.  Tin  enters  into  many  alloys,  as  the 
various  forms  of  bronze  (gun -metal  and  bell-metal,  etc.)  in 
which  it  is  alloyed  with  copper;  it  also  forms  alloys  with 
lead  in  pewter  and  several  kinds  of  solder;  with  antimony 
in  Britannia  metal;  with  both  lead  and  antimony  in 


DESCRIPTION   OF   MINERAL   SPECIES.  207 

Queen's  metal,  and  copper  and  antimony  in  Babbitt  metal; 
with  lead  and  bismuth  in  fusible  metal  (see  p.  178). 

Cassiterite,  or  Tin-stone.     Tin  dioxide,  SnOa. 

CASSITERITE,  or  tin-stone,  occurs,  when  crystallized,  in 
square  prisms  and  pyramids  and  other  related  forms;  twin 
crystals  are  common  (Fig.  180).  The  crystals  have  a 
splendent  adamantine  luster  and  a  brown  color,  sometimes 
nearly  black.  It  is  also  found  disseminated  in  irregular 
particles  in  certain  rocks,  and  there  is  a  kind  called 
stream-tin,  which  consists  of  rolled  grains  or  pebbles  of  the 
mineral;  the  massive  forms  sometimes  have  180. 

a  botryoidal  or  reniform  surface,  a  fibrous 
structure  (then  called  wood-tin],  a  brownish 
color,  and  a  dull  luster. 

Cassiterite  is  remarkable  for  its  hardness 
(6.5  to  7),  and  still  more  for  its  high  specific 
gravity,  about  7.     The  composition  is  tin  dioxide,  SnOa, 
which  when  pure  contains  78.6  per  cent  of  metallic  tin. 

Cassiterite  occurs  commonly  in  granite;  either  in  veins 
or  sprinkled  through  the  rock,  often  in  inconspicuous 
particles,  which  can  be  separated  by  the  same  process  that 
nature  has  used  in  making  stream-tin,  that  is,  after  the 
rock  has  been  crushed,  by  washing  away  the  lighter 
material  by  running  water,  the  heavy  tin-stone,  more  or 
less  pure,  being  left  behind. 

Cassiterite  is  rather  easily  recognized  when  in  large 
crystals  or  masses  by  its  high  specific  gravity,  hardness,  rich 
brown  color,  and  brilliant  luster.  But  confirmation  is 
usually  needed,  and  this  can  be  gained  by  grinding  some  of 


208  MINERALS,  AND   HOW   TO   STUDY   THEM. 

the  mineral  fine  in  an  agate  mortar,  mixing  it  with  sodiun 
carbonate,  and  patiently  roasting  it  on  charcoal.  After  a 
little  time  small  globules  of  a  white  metal  separate,  and  by 
the  method  described  on  pp.  145,  146  they  can  be  shown 
to  be  malleable  under  the  hammer,  while  they  are  harder 
than  silver,  which  they  resemble;  in  nitric  acid  they  are 
oxidized  to  an  insoluble  white  powder,  having  the  same 
composition  as  the  original  mineral. 

Cassiterite  is  found  sparingly  at  a  number  of  points  in 
the  United  States,  but  attempts  to  mine  it,  as  near  Harney's 
Peak,  S.  Dakota,  and  in  San  Bernardino  County,  Cali- 
fornia, have  not  been  successful.  It  is  obtained  in  Mexico 
rather  abundantly  in  the  state  of  Durango.  The  Cornwall 
mines  in  England  have  furnished  it  for  many  centuries,  as 
also  the  Saxon  and  Bohemian  mines.  Borneo,  Sumatra, 
Banca,  Malacca,  and  other  islands  in  the  East  Indies, 
and  further,  Australia,  yield  large  quantities  at  the 
pres«nt  time. 

TITANIUM. 

TITANIUM  is  a  rare  element,  chemically  related  to  tin;  it 
is  of  no  special  economic  importance  at  present,  though 
it  forms  certain  alloys  which  may  come  into  use  in  the 
future.  The  most  important  minerals  containing  titanium 
are  the  oxides  rutile,  octahedrite,  brookite;  also  the 
silico-titanate,  called  titanite  or  sphene;  the  last-men- 
tioned mineral  is  described  on  a  later  page.  Rutile,  octa- 
hedrite, and  brookite  have  all  the  same  composition, 
namely,  titanium  dioxide,  TiO, ,  but  they  differ  in  crystal- 
line  form. 

RUTILE  is  tetragonal,  and  OCTAHEDKITE  (see  Figs.  43,  45. 


DESCRIPTION  OF  MINERAL  SPECIES.  209 

p.  32)  is  also,  but  of  different  form,  while  BROOKITE  be- 
longs to  the  orthorhombic  system.  The  last  two  species 
are  so  rare  that  they  will  not  be  particularly  described, 
but  rutile,  though  not  common,  is  more  important. 

One  form  of  the  crystals  of  rutile  is  shown  in  Fig.  181; 
others  are  twin  crystals,  sometimes  quite  complex,  eight 
partial  crystals  occasionally  going  together  to  make  one 
compound  group.  Other  twins  are  of  the  knee-shaped 
kind,  called  geniculated,  as  shown  in  Fig.  182. 

182. 


The  hardness  is  6  to  6.5,  and  the  specific  gravity  about 
4.2.  The  color  varies  from  reddish  brown  to  red  or  yellow- 
ish, and  also  to  nearly  black,  though  even  in  the  last  variety 
thin  splinters  let  through  a  little  reddish  light.  The  luster 
is  usually  metallic-adamantine.  It  is  quite  infusible  and 
reacts  for  titanium  (see  p.  139),  and  also  most  varieties  for 
iron,  which  is  usually  present  (3  to  4  p.  c.  Fe40,). 

Rutile  is  found  chiefly  in  gneiss  or  granite,  also  in  gran- 
ular limestone.  It  is  occasionally  cut  for  mourning  jewelry. 
When  penetrating  rock-crystal  in  very  slender  transparent 
crystals  it  forms  specimens  of  great  beauty,  particularly 
when  polished;  these  are  sometimes  called  love's  arrows 
or  fleches  d'amour.  Eutile  is  also  used  to  color  porcelain 
yellow  and  to  give  a  tint  to  artificial  teeth. 


210  MINERALS,  AND    HOW   TO   STUDY   THEM. 


URANIUM. 

URANIUM  is  another  rare  element,  but  one  of  some  im- 
portance economically.  Its  compounds  have  usually  a 
bright  yellow  or  green  color,  and  a  little  present  in  glass 
gives  it  a  bright  canary-yellow  of  fluorescent  properties. 
It  is  employed  in  certain  pigments;  also  in  painting  on 
porcelain. 

The  most  important  mineral  containing  uranium  is 
URANINITE,  in  which  it  is  combined  with  oxygen;  some 
lead  and  other  rare  elements  are  also  present.  It  occurs 
rarely  in  black  octahedrons  of  very  high  specific  gravity, 
up  to  9.7,  also  commonly  in  massive  forms  having  a  pitch- 
black  color  and  luster,  and  hence  called  pitchblende.  This 
last  variety  is  more  or  less  altered  and  yields  some  water 
in  the  closed  tube. 

Two  other  uranium  minerals  are  TORBERNITE,  phos- 
phate of  uranium  and  copper,  a  mineral  of  a  bright  green 
color,  and  AUTUNITE,  phosphate  of  uranium  and  calcium, 
which  is  bright  yellow.  Both  minerals  occur  in  thin 
tabular  crystals  which  have  a  basal  cleavage,  somewhat 
resembling  mica  (though  the  scales  are  brittle),  and  hence 
they  are  sometimes  included  together  under  the  name  of 
uranium  mica. 

IRON. 

IRON  may  well  be  called  the  most  important  of  all  the 
metals.  How  large  a  place  it  takes  in  the  work  of  the 
world  is  shown  by  the  fact  that  each  year  some  50  million  or 
more  tons  are  produced  from  its  various  ores  and  turned 


DESCRIPTION   OF   MINERAL   SPECIES.  211 

into  some  of  the  many  forms  needed  by  man  in  his  work. 
Its  manifold  uses  are  too  well  known  to  need  enumer- 
ation here.  The  entire  supply  of  iron  which  the  world 
uses  each  year  is  obtained  from  its  ores,  in  which  the  iron 
is  in  combination  chiefly  with  oxygen.  These  ores  are  the 
minerals  hematite,  magnetite,  and  limonite;  siderite,  the 
carbonate  of  iron,  is  also  an  important  ore. 

These  ores  smelted,  for  example  with  limestone  as  a  flux, 
in  a  blast-furnace  with  charcoal  or  coke,  yield  pig  iron,  an 
impure  form  containing  much  carbon.  This  is  also  some- 
times called  cast  iron,*  though  now  this  name  is  chiefly 
given  to  iron,  remelted  in  a  cupola  furnace  and  cast  in 
any  desired  form,  which  is  also  rich  in  carbon.  If  purified 
so  as  to  contain  but  little  carbon,  it  becomes  wr -ought  iron 
of  very  different  properties,  while  steel  is  in  composition 
intermediate  between  the  two  forms  mentioned.  Steel  is 
now  obtained  chiefly  by  the  Bessemer  process.  It  is 
remarkable  because  of  its  great  strength  and  the  varying 
degrees  of  hardness  and  elasticity  which  can  at  will  be 
given  to  it  by  the  process  called  tempering. 

NATIVE  IRON,  or  iron  occurring  in  nature  in  the  metallic 
condition,  is  only  known  as  a  great  rarity  and  hence  is  of 
no  practical  importance.  The  meteorites  (p.  2)  which 
occasionally  fall  to  the  earth  often  consist  entirely  of 
metallic  iron,  while  others  that  have  a  stony  aspect  contain 
many  particles  of  metallic  iron  distributed  through  the 


*  Cast  iron  is  hard,  brittle,  fusible,  and  not  weldable;  wrought 
iron  is  soft,  malleable  and  ductile,  weldable  and  fusible  at  a  high 
temperature;  steel  is  malleable,  weldable  and  fusible,  with  a  varying 
hardness  depending  upon  the  temper.  (Cent.  Diet.) 


212  MINERALS,  AND   HOW  TO  STUDY  THEM. 

mass.  Native  iron  has  also  been  noted  a  few  times  in  ter- 
restrial rocks,  but  only  one  occurrence  is  especially  note- 
worthy— that  of  Disko,  Greenland,  where  it  has  been  found 
in  large  masses  imbedded  in  basalt. 

Pyrrhotite,  or  Magnetic  Pyrites.     Iron  sulphide,  Fe7S,. 

PYRRHOTITE  takes  its  name  from  a  Greek  word  meaning 
reddish  (TtvppoTrjs)  because  of  its  peculiar  reddish  bronze 
color;  this  is  a  very  important  character  to  remember. 
The  common  name,  magnetic  pyrites,  refers  to  its  still 
more  striking  character  of  being  magnetic  and  hence  at- 
tracted by  a  magnet. 

Pyrrhotite  is  rarely  found  in  hexagonal  crystals,  but  for 
the  most  part  it  occurs  in  irregular  masses.  The  hardness 
is  3.5  to  4.5,  and  the  specific  gravity  4.6.  The  luster  is 
metallic,  and  the  color,  as  before  noted,  a  peculiar  reddish 
bronze  quite  different  from  the  other  kinds  of  iron  pyrites; 
the  streak  is  dark  grayish  black. 

It  is  a  sulphide  of  iron  nearly  equivalent  to  the  simple 
sulphide,  FeS,  though  never  having  exactly  this  composi- 
tion; on  the  contrary  the  common  formula  is  Fe,S8.  On 
charcoal  it  fuses  to  a  magnetic  globule,  and  in  the  open 
tube  gives  sulphurous  fumes.  It  is  decomposed  by  hydro- 
chloric acid  with  the  separation  of  the  ill-smelling  gas 
hydrogen  sulphide. 

Pyrrhotite  often  contains  nickel,  and  though  it  is  not  usu- 
ally present  in  large  amount  (rarely  over  5  per  cent),  this 
species  occurs  so  abundantly,  for  example  at  Sudbury,  Ont., 
as  to  constitute  one  of  the  most  important  ores  of  nickel. 


DESCRIPTION   OF   MINERAL   SPECIES. 


213 


Pyrite,  or  Iron  Pyrites.     Iron  disulphide,  FeS,. 

PYRITE  is  one  of  the  commonest  and  most  striking  of 
metallic  minerals.  It  is  often  found  in  cubic  crystals  (Fig. 
183),  and  the  faces  of  these  usually  show,  if  carefully 
examined,  fine  lines  or  striations  parallel  in 
each  case  to  one  pair  of  edges  only;  further, 
on  each  face  the  direction  is  at  right  angles 
to  those  on  the  adjoining  faces.  These 
striations  have  been  explained  before  (p. 
52)  as  due  to  what  is  called  oscillatory 
combination  of  the  cubic  faces  with  those  of  the  pyrito- 
hedron.  Octahedrons  of  pyrite  are  also  common,  and  the 
twelve-sided  form  called  from  this  species  a  pyritohedron 


184. 


185. 


186. 


187. 


(Fig.  184).  These  last  sometimes  show  fine  striations  like 
the  cube,  and  often  the  two  forms  are  both  present  and 
sometimes  they  are  rounded  together.  Fig.  185  shows  the 
pyritohedron  and  cube;  Figs.  186, 187,  the 
pyritohedron  and  octahedron.  The  angle 
between  a  and  e  is  153°  26';  between  o 
and  e  140°  46'.  Pyrite  is  also  found  in 
massive  form  and  sometimes  in  large  beds 
which  can  be  mined  for  the  sake  of  the 
sulphur  which  the  ore  yields  on  roasting. 


214  MINERALS,  AND    HOW   TO    STUDY    THEM. 

The  hardness  of  pyrite  is  a  little  above  6,  so  that  it 
scratches  glass  and  is  not  scratched  by  an  ordinary  knife. 
It  is  thus  unusually  hard  for  a  sulphide,  and  it  is  due  to 
this  that  it  strikes  fire  with  the  steel,  which  is  the  source 
of  the  name  pyrites,  which  it  shares  with  some  other  hard 
sulphides  (see  the  Index).  The  specific  gravity  is  5.  The 
luster  is  brilliant,  metallic,  and  the  color  light  brass-yellow, 
sometimes  growing  a  little  deeper  when  tarnished.  The 
streak  is  dark  greenish  black. 

The  composition  is  iron  disulphide,  FeS.,,  which  gives: 
Sulphur  53.4,  iron  46.6  =  100.  Though  consisting  nearly 
one  half  of  iron,  it  is  of  no  value  as  an  iron  ore,  but  it  is 
employed  for  making  sulphur  and  sulphuric  acid;  some 
kinds  (called  auriferous  pyrites)  are  mined  for  the  small 
amount  of  gold  they  yield  when  smelted.  It  fuses  on 
charcoal  to  a  black  metallic  bead,  giving  off  sulphur  which 
burns  and  produces  the  suffocating  fumes  of  sulphur  di- 
oxide; in  the  closed  tube  the  sulphur  which  is  driven  off 
collects  in  the  cooler  part  of  the  tube  in  a  liquid  ring 
which  is  orange-red  when  hot  and  turns  sulphur-yellow  as 
it  grows  cool  and  solidifies. 

Pyrite  is,  as  has  been  stated,  a  very  common  mineral, 
forming  large  beds,  as  in  Spain,  at  Rowe,  Mass.,  and  in 
Virginia.  In  metallic  veins  it  is  almost  always  present, 
sometimes  abundantly.  It  is  also  often  found  in  crystals 
in  slates  and  many  kinds  of  rocks,  and  in  coal,  though 
then  much  diminishing  its  value  if  in  large  amount. 

Pyrite  is  often  associated  with  chalcopyrite,  or  copper 
pyrites,  but,  as  stated  on  p.  192,  it  is  easy  to  distinguish 
the  two  minerals  by  the  difference  in  hardness  and  color, 
also  by  blowpipe  characters. 


DESCRIPTION   OF  MINERAL   SPECIES.  215 

Marcasite,  or  White  Iron  Pyrites.     Iron  disulphide,  FeSa. 

MARCASITE  has  the  same  chemical  composition  as  the 
more  common  kind  of^  iron  pyrites,  called  pyrite,  but  it 
differs  in  the  form  of  the  crystals,  in  specific  gravity,  some- 
what in  color,  and  other  respects.  The  two  are  conse- 
quently distinct  minerals,  and  the  compound  FeS.,  is  said 
to  be  dimorphous  (p.  120). 

Marcasite  crystallizes  in  orthorhombic  prisms  and  pyra- 
mids, which  it  is  generally  easy  to  distinguish  from  the 
cubes  and  pyritohedrons  of  pyrite.     The 
crystals   are  often   compounded  together 
and  grouped  in   various  forms,  to  which 
some  fanciful  names  have  been  given,  as 
spear  pyrites  (Fig.  188),  cockscomb  pyrites, 
etc.      It    often  forms    nodules,  spherical 
forms,  or  stalactites,  and  is  also  simple  massive. 

The  hardness  is  6  to  6.5,  or  the  same  as  pyrite,  but 
the  specific  gravity  is  lower,  only  4.8  instead  of  5.  The 
color,  too,  is  a  paler  yellow  when  quite  fresh,  so  that  it 
is  often  called  white  iron  pyrites.  It  is,  however,  more 
easily  altered  by  the  action  of  the  weather  and  becomes 
tarnished,  the  metallic  luster  then  becomes  dull,  and  the 
true  color  is  more  or  less  obscured. 

Arsenopyrite,  or  Arsenical  Pyrites.     Iron  sulph- arsenide, 
FeSa.FeAs,. 

ARSENOPYRITE  is  another  member  of  the  pyrites  group, 
and  like  the  others  is  hard  enough  to  strike  fire  with  a 
steel.  It  is  near  to  pyrite  and  marcasite  in  composition,  but 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


besides  sulphur  contains  also  arsenic,  so  that  it  is  often 
called  arsenical  pyrites:  mispickel  is  another  name.  It  is 
found  commonly  in  masses,  but  occurs  also  in  orthorhom- 
bic  crystals,  which  are  much  like  those  of  marcasite  and 
sometimes  twinned  in  the  same  way.  The  angle  between 
the  front  m  faces  is  112°,  and  of  e  (over  the  top  edge) 
59^°,  of  u  147°,  of  q  80°. 

The  hardness  is  about  6,  and  the  specific  gravity  also  6. 
The  luster  is  metallic,  and  the  color  when  fresh  silver- 
189.  190.  101. 


white,  becoming  a  little  dull  and  tarnished  after  exposure. 
The  color  is,  therefore,  quite  different  from  the  reddish 
bronze  of  pyrrhotite  or  the  pale  yellow  of  pyrite  and  mar- 
casite. The  streak  is  grayish  black. 

The  formula  is  FeAsS,  which,  to  show  the  relation  to 
marcasite,  may  be  written  FeSa.FeAs2.  This  gives  the 
following  percentage  composition:  Sulphur  19.7,  arsenic 
46.0,  iron  34.3  =  100.  Heated  alone  on  charcoal  it  fuses  to 
a  black  magnetic  globule,  giving  off  dense  white  fumes  of 
arsenic  trioxide,  As20, ,  which  are  so  volatile  that  they  do 
not  condense  on  the  coal  except  at  a  considerable  distance 
from  the  flame.  The  touch  of  a  flame  upon  this  white 
coating  drives  it  away.  In  the  open  tube,  heated  very 
sloiuly,  the  sulphur  is  oxidized  and  passes  out  of  the  tube 
as  SO, ,  and  the  arsenic  forms  AsaOs,  which  condense0  ID  *~ 


DESCRIPTION   OF   MINERAL   SPECIES.  217 

bube  as  brilliant  spangling  octahedral  crystals— this  is  the 
poisonous  "white  arsenic"  (or  simply  "arsenic")  of  the 
druggist,  and  it  is  obtained  in  large  quantities  in  the  pro- 
cess of  roasting  this  mineral  as  well  as  arsenical  ores  of  iron 
and  cobalt,  as  in  Cornwall.  Heated  in  the  closed  tube, 
where  there  is  no  air  supplied,  a  dark  red  ring  of  arsenic 
sulphide  (As2S3)  is  first  formed ;  if  the  heating  is  continued, 
metallic  arsenic  now  goes  off,  and  collects  as  another  ring, 
which  is  in  black  and  lustrous  scales.  The  arsenopyrite  of 
Deloro,  Canada,  is  auriferous  and  hence  mined  for  the  gold 
it  yields. 

Hematite,  or  Red  Oxide  of  Iron.     Iron  sesquioxide,  Fea03. 

HEMATITE  is  named  from  the  Greek  word  for  blood 
(ai^ci},  because  many  kinds  show  a  red  color  and  all  varie- 
ties give  a  red  streak.  It  is  found  in  a  great  many  differ- 
ent forms  and  is  a  difficult  mineral,  consequently,  for  the 
beginner  to  learn  thoroughly.  The  kinds  with  a  brilliant 
metallic  luster  are  called  specular  iron;  beautiful  speci- 
mens of  this  come  from  the  island  of  Elba,  where  it  is 
found  in  thick  rhombohedral  crystals  with  brightly  pol- 
ished faces,  often  with  a  beautiful  iridiscent  tarnish  on  the 
surface.  These  crystals  are  usually  rather  complex  (Figs. 
194,  195),  but  occasionally  the  simple  form  resembling  a 
cube  in  angle  (86°)  is  observed  (Figs.  192,  193).  Other 
crystallized  kinds,  perhaps  more  common,  are  in  thin  plates 
or  scales  (Figs.  196, 197),  sometimes  so  thin  as  to  be  trans- 
parent, and  blood-red  in  color  when  looked  through.  The 
specular  iron  is  also  massive,  with  black  color  and  brilliant 
luster,  and  the  masses  have  sometimes  a  peculiar  smooth, 


218 


MINERALS,  AND    HOW   TO   STUDY   THEM. 


almost  conchoidal,  fracture;  certain  forms  have  a  reniform 
surface. 

Other  kinds  of  hematite  are  in  scales  a  little  like  mica, 
sometimes  black  and  shining,  less  often  soft  and  reddish 
and  soapy  to  the  feel;  also  in  minute  pealike  forms,  called 
fossil  ore.  An  earthy  kind,  dull  in  luster,  is  the  red  ocher, 
used  for  making  paint. 

The  hardness  of  most  kinds  of  hematite  is  about  6,  so 
193.  194. 


that  it  is  too  hard  to  be  scratched  by  the  knife;  but  some 
of  the  scaly  kinds  are  soft  and  unctuous  to  the  touch. 
The  specific  gravity  of  the  crystals  is  5.2.  The  luster  is 
metallic  in  the  specular  iron  variety,  but  dull  and  earthy 
in  others.  The  color  i8  usually  iron- black,  but  also  red. 
The  streak  is  a  dull  red,  a  little  like  that  of  dried  blood; 
the  black  micaceous  kinds  have  to  be  ground  quite  fine  to 
show  this.  Some  kinds  are  slightly  magnetic,  but  probably 
only  because  of  a  small  admixture  of  magnetite. 

Hematite  is  the  sesquioxide  of  iron,  FeaO,,  and  if  pure 


DESCRIPTION   OF   MINERAL  SPECIES.  219 

contains  70  per  cent  of  metallic  iron;  it  is  also  called  ferric 
oxide  by  the  chemist,  in  distinction  from  the  protoxide  or 
ferrous  oxide,  FeO.  Heated  in  the  reducing  flame  of  the 
blowpipe,  a  fragment  is  partially  converted  into  the  mag- 
netic oxide  so  that  a  magnet  will  pick  it  up.  Hematite  is 
the  iron  ore  mined  in  much  of  the  Lake  Superior  region, 
at  Birmingham,  Alabama,  and  elsewhere  in  the  Southern 
States,  and  formerly  at  the  famous  Iron  Mountain  of  Mis- 
souri. The  most  beautiful  crystallized  specimens  have 
come  from  the  island  of  Elba;  Switzerland  and  France 
also  afford  fine  crystals. 

Magnetite  or  Magnetic  Oxide  of  Iron.     Fe(FeJ04. 

MAGNETITE  suggests  in  its  name  its  most  striking  char- 
acter, that  of  being  magnetic.  All  kinds  are  strongly  at- 
tracted by  a  magnet,  and  one  variety,  called  the  lodestone, 


found  for  example  at  Magnet  Cove,  Arkansas,  is  a  powerful 
magnet  itself.  It  has  a  north  and  south  pole,  the  power  of 
picking  up  particles  of  iron  or  steel,  as  tacks,  and  also,  when 
suspended,  it  sets  with  its  poles  north  and  south  like  a 
compass-needle. 


MINERALS,  AND   HOW   TO    STUDY   THEM. 


Magnetite  is  found  in  octahedral  or  dodecahedral  crys- 
tals (Figs.  199-201);  more  commonly  simply  massive,  and 
then  sometimes  with  a  peculiar  fracture,  suggesting  cleav- 
age, yielding  octahedral  fragments. 

The  hardness  is  high,  about  6,  like  that  of  hematite,  and 
the  specific  gravity  is  nearly  the  same,  5.18.  The  luster  is 
200.  201. 


metallic,  usually  very  brilliant,  and  the  color  iron-black. 
The  streak  also  is  black,  and  this  is  a  most  important  char- 
acter, for  it  distinguishes  it  at  once  from  hematite,  which, 
though  at  times  iron-black  in  the  mass,  has  a  red  streak. 

The  composition  is  expressed  by  the  formula  Fe(Fea)04 
or  FeO.Fea03,  which  is  equivalent  to  Fe304,  yielding  72.4 
per  cent  of  metallic  iron.  It  is  fused  with  great  difficulty, 
but  a  small  fragment  heated  carefully  in  the  oxidizing 
flame  loses  part  if  not  all  of  its  magnetic  property.  It  is 
soluble  in  hydrochloric  acid. 

Magnetite,  like  hematite,  is  a  very  important  ore  of  iron. 
It  has  been  mined  in  the  Adirondack  region,  as  at  Port 
Henry,  in  large  quantities  and  elsewhere;  also  in  the  West 
Point  region;  at  Brewster,  Putnam  County;  in  New  Jersey. 
It  also  occurs  in  the  Lake  Superior  region,  where,  however, 
the  commoner  ore  is  hematite.  The  famous  Swedish  iron 
and  steel  are  made  from  magnetite.  Besides  these  great 
deposits  magnetite  is  a  common  mineral  in  many  rocks, 


DESCRIPTION   OF   MINERAL   SPECIES.  221 

occurring  in  little  particles  distributed  through  the  mass. 
It  is  thus  prominent  in  the  trap  rocks  of  Connecticut, 
Massachusetts,  and  the  Palisades  of  the  Hudson.  When 
the  rocks  containing  magnetite  are  broken  up  by  the 
weather  and  reduced  to  the  condition  of  sand  and  gravel, 
the  magnetic  iron,  being  heavier,  is  often  sorted  out  by  the 
water  and  accumulated  by  itself;  a  stream  by  the  side 
of  a  country  road  often  shows  a  streak  of  the  black  iron 
sand,  and  at  the  seashore  it  may  be  found  in  quite  large 
quantities.  It  has  been  mined  in  this  way  at  Block  Island, 
being  separated  by  a  large  magnet  from  the  associated 
sand  and  gravel. 

FRAXKLINITE,  so  called  from  its  sole  locality,  Franklin 
Furnace,  New  Jersey,  is  a  mineral  in  form,  color,  and  gen- 
eral appearance  much  resembling  magnetite,  but  it  is  only 
feebly  magnetic  if  at  all,  and  has  a  brown,  not  black,  streak. 
It  is  an  oxide  containing  besides  iron  also  zinc  and  man- 
ganese, and  is  hence  valuable  as  a  zinc  ore  and  for  making 
spiegeleisen,  an  alloy  of  iron  and  manganese  employed  in 
the  making  of  steel. 

CHROMITE,  or  Chromic  Iron,  is  another  iron  ore  looking 
much  like  magnetite,  also  crystallizing  in  octahedrons, 
though  the  massive  form  is  the  common  one.  It  contains 
chromium  besides  iron,  and  with  borax  yields  a  chrome- 
green  bead  (p.  138).  It  is  not  a  particularly  interesting 
mineral  and  is  of  limited  occurrence,  but  valuable  as  a 
source  of  the  element  chromium,  which  forms  a  bright- 
colored  (usually  yellow  or  green)  class  of  salts  called  chro- 
mates;  these  are  used  for  pigments  and  in  calico-printing. 
Chromium  is  also  used  in  chrome-steel.  It  is  often  as- 


222  MINERALS,  AND   HOW   TO   STUDY  THEM. 

sociated  with  serpentine,  as  in  Pennsylvania   and  Mary- 
land ;  it  is  also  mined  in  California  and  in  Turkey. 

ILMENITE,  or  titanic  iron,  is  related  to  hematite  and 
magnetite,  but  differs  from  the  former  in  having  a  black 
streak,  and  from  the  latter  in  not  being  magnetic.  It  con- 
tains titanium  besides  iron  and  oxygen,  and  the  formula  of 
some  kinds  isFeTi04.  Like  magnetite  it  occurs  in  minute 
particles  in  certain  rocks  and  it  also  forms  beds  of  some 
magnitude.  Part  of  the  magnetite  contains  titanium  (or 
is  titaniferous)  and  is  then  of  much  less  value  as  an  ore 
because  highly  refractory,  or  hard  to  reduce  in  a  furnace. 

Limonite,  Brown  Oxide  of  Iron.     2Fea08.3H30. 

LIMONITE  is  a  hydrous  oxide  of  iron,  that  is,  it  contains 
some  14  per  cent  of  water  which  it  gives  off  when  heated. 
It  is  often  called  brown  hematite,  because  while  re- 
sembling some  kinds  of  hematite  it  has  usually  a  brown 
color  and  always  a  brown  streak.  It  is  not  known  in  crys- 
tallized forms,  but  occurs  only  massive,  especially  in  stalac- 
titic  shapes,  or  forms  with  rounded  surface  (see  Fig.  140, 
p.  68).  Its  structure  is  frequently  fibrous,  but  earthy  in 
the  brown  ocher  used  for  paint. 

The  hardness  in  the  compact  kinds  is  about  5,  or  less 
than  that  of  hematite  and  magnetite,  and  the  specific 
gravity  is  a  little  below  4.  The  luster  varies  from  sub 
metallic  to  earthy;  it  is  sometimes  brilliant  on  the  glossy 
surfaces  of  stalactites,  but  more  commonly  dull. 

It  is  a  hydrated  oxide,  2FeaOs.3H.10,  and  only  contains 
60  p.  c.  of  metallic  iron  if  perfectly  pure,  which  is  rarely 
the  case.  Heated  in  the  closed  tube  considerable  water  is 


DESCRIPTION   OF    MINERAL  SPECIES.  223 

given  off  which  condenses  in  the  colder  part  of  the  tube; 
the  fragment  after  heating  turns  red  and  has  a  red  streak; 
— by  the  loss  of  water  it  has  been  converted  into  an- 
hydrous iron  sesqnioxide,  Fe20s,  or  hematite. 

Limonite  is  named  from  the  Greek  word  (XeijJGOv) 
meaning  meadoir,  because  often  found  in  marshy  places; 
in  fact  one  kind  is  also  called  bog  iron  ore.  It  is  mined 
in  many  deposits  in  western  New  England  and  adjacent 
parts  of  New  York,  also  in  Pennsylvania,  Virginia,  etc. 
It  is  a  low-grade  ore,  that  is,  it  yields  only  a  relatively 
small  amount  of  iron  because  of  the  clay  and  other  im- 
purities present. 

GOETHITE,  named  after  the  poet  Goethe,  is  another 
oxide  of  iron  yielding  water.  It  occurs  in  brilliant  pris- 
matic crystals  and  also  in  massive  forms,  often  fibrous  in 
structure.  It  has  a  yellow-brown  to  deep  brownish-black 
color  and  a  streak  like  that  of  limonite,  which  it  also  re- 
sembles in  some  of  its  forms.  It  is  of  limited  occurrence. 

TURGITE  is  still  another  iron  hydrate,  not  common;  it 
occurs  in  forms  like  limonite,  but  yields  only  5  per  cent 
of  water  and  has  a  red  streak;  it  usually  decrepitates 
when  heated  before  the  blowpipe. 

Siderite,  or  Spathic  Iron.     Iron  carbonate,  FeCOs. 

SIDERITE,  the  carbonate  of  iron,  is  also  an  important 
ore,  although  less  so  than  the  three  prominent  oxides.  It 
crystallizes  in  rhombohedrons,  often  with  rounded  faces 
(Fig.  202),  and  has  perfect  rhombohedral  cleavage.  This 
cleavage  is  a  very  prominent  character  in  the  common 
massive  kinds;  the  angle  between  two  adjacent  cleavage 


224  MINERALS,  AND    HOW  TO   STUDY  THEM. 

surfaces  is  107°  or  73°;  it  is  the  same  form  that  we  shall 
learn  with  calcite;  indeed,  as  explained  on  p.  119,  the  two 
203  species  form  with  several  others  an  isomor- 

phous  group.  The  hardness  is  ;J.;"i  to  4,  and 
the  specific  gravity  is  rather  high,  3.8;  this 
instantly  suggests  to  one  picking  up  a  speci- 
men that  a  heavy  metal  is  present.  The 
luster  is  vitreous  and  the  color  light  yellow  to  brown,  be- 
coming dark  by  alteration ;  the  streak  is  white  or  nearly  so. 
It  is  a  carbonate  of  iron,  FeC03,  and  contains  48  per 
cent  of  metallic  iron  if  pure.  In  acid,  if  slightly  warmed, 
it  dissolves  with  effervescence,  giving  off  carbon  dioxide 
gas;  before  the  blowpipe  it  turns  black,  fuses,  but  not 
very  easily,  and  becomes  magnetic.  It  is  largely  mined  in 
Cornwall,  and  is  also  found  in  Pennsylvania,  Ohio,  etc. 

COLUMBITE  is  a  rare  iron  mineral,  occasionally  found  in 
jet-black  crystals  or  masses  in  the  granite  veins  of  New 
England;  it  resembles  tourmaline  somewhat,  but  is  much 
denser,  having  a  specific  gravity  varying  from  5.4  to  6  or 
over.  Its  luster  is  submetallic.  A  figure  of  a  twin  crys- 
tal is  given  on  p.  58  (Fig.  118).  It  is  a  niobate  (or 
columbate)  of  iron  with  also  some  manganese,  ard  further 
with  the  niobium  there  are  also  present  varying  amounts 
of  the  related  element  tantalum;  as  this  increases  the 
specific  gravity  runs  up  to  about  7.  TANTALITE  is  nearly 
pure  iron  tantalate,  with  G.  =  7. 

SAMARSKITE  is  a  velvet-black  mineral  a  little  resembling 
columbite  and  often  associated  with  it.  Besides  iron  it 
contains  tantalum,  niobium,  the  cerium  metals,  yttrium, 


DESCRIPTION   OF   MINERAL   SPECIES.  225 

and  other  rare  elements;  it  is  found  in  the  mica  mines  of 
North  Carolina. 

WOLFRAMITE,  a  tungstate  of  iron  and  manganese,  is  a 
still  rarer  mineral  than  columbite.  It  is  iron-black  in 
color,  with  fine  cleavage  and  submetallic  luster;  like 
columbite  it  is  very  heavy,  having  a  specific  gravity  of 
over  7. 

TRIPHYLITE  is  a  rather  rare  phosphate  of  lithium  and 
iron  chiefly  (LiFePOJ,  but  containing  also  manganese  and 
hence  passing  into  LITHIOPHILITE  (LiMnPOJ.  It  occurs 
in  cleavable  masses  of  a  bluish-gray  color;  lithiophilite  is 
salmon  color,  yellow  or  pale  brown.  Both  minerals  have 
similar  physical  characters :  hardness  4.5-5;  specific  grav- 
ity 3.5;  luster  resinous.  In  the  forceps  they  fuse  readily, 
giving  a  red  flame  (lithium),  with  bluish  green  on  the 
edge  (phosphorus);  they  give  with  borax  reactions  for 
iron  or  manganese  or  both.  CHILDRENITE  is  essentially  a 
hydrated  phosphate  of  iron  and  alumina,  occurring  in 
yellow  or  brown  orthorhombic  crystals.  EOSPHORITE  is 
the  closely-related  manganese  compound. 

VIVIANITE  is  a  hydrated  phosphate  of  iron  having  a 
blue  to  green  color;  it  occurs  in  crystals,  also  in  earthy 
forms,  the  latter  called  blue  iron  earth. 

PHARMACOSIDERITE  is  a  hydrated  arsenate  of  iron  com- 
monly occurring  in  small  cubic  crystals  of  a  yellow  to 
green  color.  SCORODITE  is  another  arsenate  of  iron 
which  is  found  in  olive-green  to  brown  orthorhombic  crys- 
tals, also  in  aggregations.  There  are  numerous  other 
arsenates  and  phosphates  of  iron,  but  too  rare  to  be  in- 
cluded here.  Among  the  sulphates  of  iron  may  be  men- 


226  MINERALS,  AND   HOW  TO   STUDY   THEM. 

tioned  MELANTERITE,  also  called  iron  vitriol  and  copperas, 
a  mineral  which  has  usually  been  derived  from  the  decom- 
position of  pyrite  or  marcasite.  Copperas  is  employed  in 
making  ink,  also  much  used  by  dyers  and  tanners. 

NICKEL. 

NICKEL,  though  formerly  a  little-used  metal,  has  become 
of  much  wider  application  in  recent  years.  It  is  exten- 
sively employed  now  to  plate  many  articles  of  steel — as 
knives,  scissors,  skates,  etc. — because  unlike  the  steel  it 
does  not  tarnish  or  rust  rapidly  in  the  air.  It  is  much 
used  also,  when  alloyed  with  copper,  for  small  coins,  as  tin- 
"nickels"  or  five-cent  pieces  of  this  country,  and  similarly 
in  Switzerland,  Germany,  and  Belgium.  Nickel  steel  1ms 
been  found  to  be  remarkably  strong  in  withstanding  the 
blow  of  a  cannon-ball.  The  white  alloy  called  "  German 
silver"  contains  copper,  zinc,  and  nickel  in  about  the  pro- 
portions of  5 :  3 :  2. 

There  are  not,  however,  many  minerals  which  contain 
nickel.  One  of  these  is  the  sulphide  millerite;  another  is 
niccolite,  or  nickel  arsenide.  There  are  also  some  other 
rare  compounds  of  nickel  with  sulphur,  ai'senic,  or  anti- 
mony. Nickel  is  also  present  in  some  varieties  of  the 
sulphide  of  iron,  magnetic  pyrites  or  pyrrhotite,  and  this 
occurs  in  so  large  an  amount  as  to  be  an  important  source 
of  the  metal.  There  are  further  some  hydrous  silicates 
containing  nickel,  which  are  extensively  mined  at  the 
present  time.  It  is  interesting  to  note,  finally,  though  a 
matter  of  no  practical  importance,  that  the  iron  of  meteor- 
ites is  almost  always  an  alloy  of  iron  and  nickel,  the  latter 


DESCRIPTION  OF   MINERAL   SPECIES.  227 

metal  being  present  to  the  amount  of  5  to  10  per  cent,  and 
in  rare  cases  much  more. 

MiUerite.     Nickel  sulphide,  NiS. 

MILLERITE,  the  sulphide  of  nickel,  is  remarkable  among 
minerals  because  of  its  occurrence  in  very  fine  hairlike, 
or  capillary,  forms.  These  sometimes  resemble  a  wad  of 
hair,  as  in  the  geodes  in  the  St.  Louis  limestone,  or  they 
may  be  simply  a  tuft  of  extremely  delicate  radiating 
crystals  as  in  cavities  of  hematite  at  Antwerp,  N.  Y. 
There  are  also  thin  crusts  with  fibrous  structure,  as  those 
from  Pennsylvania.  The  hardness  is  a  little  over  3,  and 
the  specific  gravity  5.6.  It  has  a  metallic  luster  and  a 
color  like  that  of  yellow  bronze,  often  slightly  tarnished; 
the  streak  is  greenish  black. 

The  composition,  NiS,  gives  64.6  per  cent  of  metallic 
nickel.  Before  the  blowpipe  millerite  reacts  for  sulphur, 
like  the  sulphides,  and  after  roasting  off  the  sulphur  a  small 
fragment  will  give  with  borax  in  the  oxidizing  flame 
a  characteristic  violet  bead  (p.  138).  The  globule  obtained 
on  charcoal,  after  heating  in  the  reducing  flame,  is  attracted 
by  a  magnet,  for  nickel  is  a  magnetic  metal  like  iron, 
though  of  much  feebler  intensity. 

It  is  only  rarely  that  millerite  occurs  in  sufficient  quan- 
tity to  be  useful  as  an  ore  of  nickel.  An  iron-nickel 
sulphide  called  PENTLANDITE  is  more  important;  POLYDY- 
MITE  is  another  nickel  sulphide. 

Niccolite.    Nickel  arsenide,  NiAs. 
NICCOLITE  is  often  called  copper-nickel,  but  not  because 
it  contains  copper,  but  only  from  its  conspicuous  pale  cop- 


228  MINERALS,    AND   HOW  TO   STUDY   THEM. 

per-red  color.  It  is  found  in  masses  of  metallic  luster  ; 
hardness  5  to  5.5,  and  specific  gravity  7.3  to  7.6.  The 
composition  NiAs  gives:  Arsenic  56.1,  nickel  43.9  —  100. 

BREITHAUPTITE  is  a  related  mineral,  rarer  than  niccolite, 
though  somewhat  resembling  it.  It  is  an  antimonide  of 
nickel,  NiSb. 

Nickel  is  also  present  with  cobalt  in  the  mineral  smalt- 
ite. 

GENTHITE  and  GARNIERITE  are  hydrous  silicates  of 
nickel  and  magnesium  of  varying  composition,  having  a 
bright  green  color.  They  are  found  only  in  massive  forms 
and  are  not  very  interesting  as  minerals,  but  highly  valu- 
able as  ores.  Garnierite  is  ruined  extensively  in  the  French 
penal  colony  of  New  Caledonia. 

COBALT. 

COBALT  is  a  metal  related  to  nickel  and  often  associated 
in  nature  with  it  in  its  various  compounds,  though  of  much 
more  limited  occurrence. 

Cobalt  minerals  are  rather  rare;  they  include  the  sul- 
phide, LINN^ITE;  the  arsenide,  SMALTITE,  which  is  the 
chief  ore;  the  sulph-ursenides,  COBALTITE,  and  GLAUCODOT. 
All  these  have  a  tin-white  color  like  that  of  arsenopyrite, 
which  also  has'  a  variety,  called  danaite,  which  contains 
cobalt.  There  is  also  a  bright  rose-red  mineral  called 
ERYTHRITE,  or  cobalt  bloom,  which  is  an  arsenate  of  cobalt. 
An  impure  oxide  of  cobalt  is  a  black  earthy  mineral. 

Cobalt,  as  a  metal,  is  not  used  in  the  arts,  but  its  salts, 
which  are  mostly  brightly  colored,  have  some  applications. 
From  the  change  in  color  that  some  of  them  undergo  on 


DESCRIPTION   OF   MINERAL   SPECIES.  229 

heating  and  losing  water  depends  their  use  as  sympathetic 
ink.  Cobalt  glass,  called  smalt,  has  a  beautiful  blue 
ultramarine  color  and  ground  up  is  used  as  a  pigment. 

MANGANESE. 

MANGANESE  is  a  metal  which  is  closely  allied  to  iron  in 
physical  characters  and  chemical  relations.  As  obtained 
by  the  chemist,  for  it  does  not  occur  in  nature,  it  is  hard 
and  brittle;  it  has  a  grayish-white  color,  and  a  specific 
gravity  of  about  8.  Like  iron  it  forms  numerous  natural 
compounds,  but  they  do  not  find  many  applications  in  the 
arts.  The  alloys  of  manganese  with  iron,  called  spiegelei- 
sen  and  ferromanganese,  are,  however,  employed  in  large 
quantities  in  making  steel,  and  most  of  the  manganese 
mined  is  used  in  this  way. 

The  common  ores  of  manganese  are  the  oxides,  pyro- 
lusite  and  manganite;  the  silicate  and  carbonate  are  beau- 
tiful minerals,  but  relatively  rare,  as  is  still  more  true  of  the 
other  natural  compounds. 

Pyrolusite.  Manganese  dioxide,  MnOa. 
PYROLUSITE  is  an  oxide  of  manganese,  MnOa,and  be- 
cause of  the  large  amount  of  oxygen  that  it  contains  it  is 
sometimes  used  in  the  laboratory  as  a  source  for  that  gas. 
The  glass-maker  also  employs  it  to  take  out  the  color  o.! 
glass,  because  the  oxygen  which  it  yields  forms  colorless 
compounds  in  it.  On  this  account  it  takes  its  name,  from 
the  Greek  words  meaning  fire  (nvp)  and  to  wash  (Xva)). 
The  French  have  a  similar  name — they  called  it  "glass- 
maker's  soap."  It  is  also  used  as  an  oxidizing  agent  in 
making  paints,  varnishes,  etc. 


230  MINERALS,    AND    HOW  TO   STUDY   THEM.       • 

It  is  a  very  soft  mineral,  soiling  the  fingers;  it  has  a 
grayish-black  color,  a  black  streak,  and  metallic  luster.  It 
usually  occurs  in  fibrous  masses,  less  often  crystallized, 
also  sometimes  in  stalactitic  forms  and  in  incrustations. 

The  composition  of  pyrolusite  is  essentially  manganese 
dioxide,  MnOa ,  but  it  also  commonly  contains  some  water. 
The  strictly  anhydrous  manganese  dioxide  is  the  mineral 
polianite,  crystallizing  in  tetragonal  forms  similar  to  crys- 
tals of  rutile  (TiOJ  and  cassiterite  (SnOa).  The  reactions 
of  manganese  with  the  fluxes  are  given  on  p.  138.  Heated 
in  the  closed  tube  pyrolusite  yields  oxygen  which  causes  a 
match  if  still  glowing  when  inserted  to  start  into  flame; 
when  treated  with  hydrochloric  acid  chlorine  is  liberated. 
Pyrolusite  is  the  common  ore  of  manganese  and  is  mined 
in  large  quantities  in  Virginia,  Georgia,  New  Brunswick, 
etc.  It  has  ordinarily,  perhaps  always,  been  formed  from 
the  related  mineral  manganite. 

Mangamte.     Manganese  hydrate,  Mna08.H,0. 

MANGANITE  is  another  oxide  of  manganese;  it  occurs 
in  brilliant  orthorhombic  prismatic  crystals  and  in  fibrous 
radiated  masses.  The  hardness  is  4,  and  the  specific  grav- 
ity 4.2  to  4.4;  the  luster  is  metallic,  and  the  color  dark 
steel-gray  to  nearly  iron-black,  while  the  streak  is  dark 
reddish  brown,  thus  distinguishing  it  from  pyrolusite,  the 
streak  of  which  is  black. 

The  formula  MnO(OH)  or  Mna03.H.,0  gives  the  per- 
centage composition:  Manganese  sesquioxide  89.7  (or  man- 
ganese 62.4),  water  10.3  =  100.  This  is  a  not  uncommon 
mineral  in  the  Lake  Superior  iron  region;  it  is  mined  in 


DESCRIPTION   OF   MINERAL   SPECIES.  23\ 

the  Harz  in  Germany.     By  loss  of  water  and  oxidation  it 
is  converted  into  pyrolnsite. 

BRAUXITE  and  HAUSMANNITE  are  other  oxides  of  man- 
ganese of  rather  rare  occurrence.  PSILOMELANE  is  com- 
moner, but  not  often  found  in  a  state  of  purity;  it  usually 
occurs  in  black  botryoidal  or  stalactitic  forms,  often  asso- 
ciated with  pyrolusite.  It  consists  chiefly  of  manganese 
oxide  and  water,  with  some  baryta,  etc.  WAD,  or  bog 
manganese,  is  a  still  less  definite  mineral,  consisting  of 
mixtures  of  oxides  of  manganese  and  other  metals  (cobalt, 
lead,  etc.).  It  is  brown  to  black  in  color,  dull  in  luster; 
very  soft,  and  often  extremely  porous  and  light,  sometimes 
sufficiently  so  to  float  on  water.  It  is  used  as  a  paint. 

Rhodonite.     Manganese  silicate,  MnSiO,. 

RHODONITE  is  named  from  the  Greek  word  for  rose 
(podov),  which  alludes  to  its  beautiful  rose-red  color.  It 
is  not  a  common  mineral,  but  is  found  rather  abundantly 
in  some  localities,  as  at  Franklin  Furnace,  N.  J.,  also  in 
Russia,  where  it  is  used  as  an  ornamental  stone,  in  the 
form  of  a  veneering  for  table-tops,  etc.  The  crystals  are 
flat  and  usually  show  two  cleavages;  they  belong  to  the 
triclinic  system,  and  the  form  is  not  easily  deciphered. 

The  hardness  is  about  6,  and  the  specific  gravity  3.6. 
The  luster  is  vitreous,  or  pearly  on  the  cleavage  faces;  the 
color  is  rose-pink;  the  streak  is  white. 

The  formula,  MnSi03,  corresponds  to  the  percent- 
age compositior :  Silica  (SiOJ  45.9,  manganese  protoxide 
(MnO)  54.1  =  100.  Some  varieties  contain  zinc,  others 
iron,  and  others  also  lime.  It  fuses  rather  easily  before 


233  MINERALS,    AND   HOW   TO   STUDY   THEM. 

the  blowpipe,  turning  black;  with  borax  it  gives  a  man- 
ganese reaction  (p.  138).  It  is  partially  dissolved  by 
hydrochloric  acid. 

Khodochrosite.     Manganese  carbonate,  MnCO,. 

EHODOCHROSITE,  the  carbonate  of  manganese,  is  another 
rose-colored  mineral  resembling  rhodonite  in  color  as  in 
its  name.  It  sometimes  occurs  in  fine  clear  rhombohe- 
drons,  and  in  masses  with  rhombohedral  cleavage,  and 
then  the  form  is  found  to  be  very  near  that  of  calcite  and 
siderite,  to  which  it  is  closely  related  (see  p.  119);  the 
angle  between  two  cleavage  faces  is  107°.  It  also  occurs 
in  massive  forms,  sometimes  granular  and  compact;  also 
globular  or  botryoidal.  The  hardness  is  about  4,  and  the 
specific  gravity  3.6.  The  luster  is  vitreous,  and  the  color 
rose-pink;  the  streak  is  white. 

The  formula,  MnC03,  requires:  Carbon  dioxide  (C02) 
38.3,  manganese  protoxide  (MnO)  61.7  =  100.  It  effer- 
vesces with  acid  and  reacts  for  manganese  with  the  fluxes 
(p.  138).  Beautiful  clear  rhombohedral  crystals  come 
from  Lake  County,  Colorado.  It  is  the  gangue  of  silver 
and  gold  ores  in  Montana,  near  Butte  City.  It  is  mined 
in  "Wales  and  in  Belgium. 

Of  the  many  other  manganese  minerals  may  be  men- 
tioned the  sulphides  ALABANDITE  (MnS)  and  HAUERITE 
(MnS2);  further,  the  phosphate  TRTPLITE  (also  triphylite 
and  lithiophilite,  p.  225);  there  are  a  number  of  other 
rare  phosphates. 


DESCRIPTION   OF   MINERAL  SPECIES.  233 

ZINC. 

!ZiNC  is  one  of  the  most  common  and  important  of  the 
metallic  elements,  but  it  is  not  certainly  known  to  occur 
in  the  form  of  the  metal  in  nature.  It  has  a  crystalline 
structure  like  metallic  antimony,  a  white  color,  and  brill- 
iant luster,  soon,  however,  tarnishing.  Its  specific  grav- 
ity is  6.9  to  7.2.  It  is  brittle  at  both  low  and  high 
temperatures,  but  at  140°  Centigrade  it  can  be  rolled  into 
sheets.  It  fuses  at  a  relatively  low  temperature,  500°  C., 
and  boils  at  a  red  heat.  Its  physical  properties  put  it 
somewhat  near  the  imperfect  metal  antimony. 

It  is  a  most  important  metal  in  the  arts.  Iron  in  sheets 
and  wire,  coated  by  zinc,  are  protected  from  rusting,  and  are 
then  said  to  be  galvanized;  a  common  use  of  the  sheets  is 
for  roofing.  Zinc  is  the  negative  metal  in  almost  all  forms 
of  the  chemical  electric  battery — that  is,  the  metal  at  the 
expense  of  which  the  electric  current  is  obtained.  With 
copper  it  forms  brass  and  related  alloys;  it  is  also  one  of 
the  constituents  in  german  silver;  an  alloy  of  zinc  is  used 
for  making  raised  cuts  in  photo-engraving.  The  white 
oxide  is  used  for  paint.  Metallic  zinc,  as  obtained  from 
the  furnace  in  ingots,  is  called  spelter. 

The  commonest  ore  of  zinc  is  the  sulphide,  sphalerite 
or  zinc  blende,  but  the  silicates,  vvillemite  and  calamine, 
and  the  carbonate,  smithsonite,  are  also  important  and 
valuable. 

Sphalerite.     Zinc  sulphide,  ZnS. 

SPHALERITE  is  named  from  a  Greek  word  which  means 
deceiving,  and  the  young  mineralogist,  after  he  has  blun- 


234  MINERALS,  AND    HOW   TO   STUDY   THEM. 

dered  over  it  a  score  of  times,  as  he  is  pretty  sure  to  do, — 
for  it  is  far  from  easy  to  recognize, — will  think  it  well 
named.  It  was  so  called  because  often  occurring  with 
and  mistaken  for  the  more  easily  recognized  lead  ore, 
galena;  the  miner's  names,  black  jack,  false  lead,  false 

203.  204. 


galena,  refer  to  the  same  fact.  The  common  name,  blende 
or  zinc  blende,  will  perhaps  be  easier  to  remember  at  first 
than  sphalerite. 

It  is  sometimes  found  in  tetrahedral  crystals  and  related 
forms  (Figs.  201,  204),  but  usually  the  crystals  are  indis- 
tinct, being  not  infrequently  twinned,  and  it  needs  a 
trained  and  skillful  eye  to  understand  them.  Usually  it  is 
found  in  masses  or  small  particles,  showing  smooth  sur- 
faces of  cleavage,  which  is  found  on  examination  to  be 
dodecahedral,  since  the  angle  between  two  adjacent  sur- 
faces is  120°.  Sometimes  it  is  possible  to  cleave  out  an 
almost  perfect  dodecahedron  from  a  mass  of  sphalerite. 
Even  if  granular  in  structure  the  cleavage  surfaces  are 
usually  prominent,  though  there  are  kinds  which  are 
closely  compact  and  show  no  cleavage. 

The  hardness  is  3.5  to  4,  and  the  specific  gravity  about 
4.  When  perfectly  pure,  sulphide  of  zinc  is  white  in  the 
form  of  powder,  or  clear  and  nearly  colorless  in  small 


DESCRIPTION   OF   MINERAL   SPECIES.  235 

cleavage  pieces  ;  the  latter  then  show  an  adamantine 
luster.  Commonly  it  contains  some  iron,  and  often  a 
good  deal,  and  then  the  color  is  yellow  or  yellowish  brown, 
— the  latter  the  most  common, — and  finally  dark  brown 
and  nearly  or  quite  black;  the  light-colored  kinds  may 
also  have  a  greenish  tinge.  The  luster  is  usually  resinous; 
and  in  all  the  common  kinds  this  is  so  distinct  that  the 
mineralogist  comes  to  depend  upon  it  to  enable  him  to 
identify  the  mineral.  The  streak  is  white,  pale  yellow,  or 
brownish,  becoming  deeper  the  darker  the  color  of  the 
mass. 

The  composition  zinc  sulphide,  ZnS,  gives:  Sulphur  33. 
zinc  67  =  100.  As  stated  above,  iron  is  usually  present, 
and  sometimes  also  manganese  and  the  rare  element 
cadmium.  Before  the  blowpipe  it  does  not  fuse,  but  if 
powdered  and  heated  on  charcoal  (see  p.  143)  it  gives  a 
zinc  coating,  canary-yellow  when  hot,  but  white  on  cool- 
ing; this  turns  green  when  heated  in  the  oxidizing  flame 
after  being  moistened  with  nitrate  of  cobalt.  When 
warmed  in  a  test-tube  with  hydrochloric  acid  it  effervesces, 
giving  off  bubbles  of  gas  which  a  careless  observer  might 
take  for  carbon  dioxide,  only  the  disagreeable  odor  shows 
that  it  is  sulphureted  hydrogen  (HSS). 

Zinc  blende  is  one  of  the  commonest  of  the  metallic 
compounds,  and  where  we  find  galena  or  pyrite  we  are 
likely  to  find  it  also;  in  one  variety  it  is  compact,  alter- 
nating in  layers  with  cleavable  galena.  It  occurs  in  some 
regions,  as  in  southwestern  Missouri  and  the  adjoining 
portions  of  Kansas,  in  very  large  deposits. 

Zincite,  Franklinite,  and  Willemite  are  all  rare  minerals, 


236  MINERALS,  AND   HOW   TO   STUDY  THEM. 

but  as  they  are  important  ores  of  zinc  at  that  famous 
locality,  Franklin  Furnace,  New  Jersey,  they  will  be  men- 
tioned briefly;  they  are  indeed  known  at  only  a  few  other 
places. 

ZINCITE,  or  the  red  oxide  of  zinc  (ZnO),  is  often  found 
in  bright  red  grains  or  masses,  sometimes  mixed  up  with 
the  other  two  minerals  named,  the  black  franklinite  and 
the  green  willemite.  It  also  occurs  in  larger  masses  in 
calcite,  and  then  shows  good  cleavage.  Hexagonal  crystals 
are  very  rare.  The  hardness  is  4  to  4.5,  and  the  specific 
gravity  5.4  to  5.7.  The  deep  red  or  orange  color  is  very 
characteristic;  the  streak  is  orange-yellow.  It  reacts  for 
zinc  on  charcoal  and  for  manganese  with  borax  on  the 
platinum  wire. 

FRANKLINITE  is  an  oxide  of  zinc,  manganese,  and  iron; 
it  has  been  already  mentioned  on  p.  221. 

GAHNITE,  or  zinc  spinel,  is  related  to  franklinite.  It 
has  often  an  octahedral  form  and  a  deep  green  color.  The 
hardness  is  7.5  to  8,  and  the  specific  gravity  4.6.  The 
typical  composition  is  ZnO.Al403 .  but  kinds  from  different 
localities  vary  widely. 

WILLEMITE  is  often  found  in  bright  yellow  or  apple- 
green  masses,  also  in  six-sided  crystals  usually  of  a  flesh-red 
color  and  which  are  sometimes  quite  large  and  have  a 
resinous  luster;  this  last  kind  is  called  troostite.  Rarely 
slender  prisms  of  a  clear  green  are  found.  The  hardness 
is  5.5,  and  the  specific  gravity  3.9  to  4.18.  The  luster  is 
between  vitreous  and  resinous,  often  weak,  and  the  color 
varies  widely,  as  already  stated. 

Willemite  is  a  silicate  of  zinc,  Zn,Si04,  or  2ZnO.SiO,,, 


DESCRIPTION   OF   MINERAL   SPECIES.  237 

and  the  percentage  composition  is:  Silica  (SiO,)  27.0, 
zinc  protoxide  (ZnO)  73.0  =  100.  Manganese  and  iron 
are  often  present,  replacing  part  of  the  zinc. 

CALAMINE  is  another  silicate  of  zinc,  but  different  from 
willemite,  since  it  contains  considerable  water,  which  it 
gives  off  when  heated  to  a  high  temperature  in  the  closed 
tube.  It  is  not  very  often  found  in  isolated  crystals,  but 
usually  in  masses  with  a  crystalline  surface,  which  is  mam- 
millary  or  botryoidal  in  form.  Occasionally  the  surface  is 
seen  to  be  made  up  of  flat  tabular  crystals  projecting  from 
the  mass.  The  hardness  is  4.5  to  5,  and  the  specific  gravity 
3. 4  to  3.5.  It  is  usually  white  or  slightly  yellowish,  but  may 
be  tinged  blue  from  a  little  copper;  the  luster  is  vitreous. 

The  composition  is  HaZnaSi06  or  H,0.2ZnO.SiOs, 
which  gives:  Silica  (SiOa)  25.0,  zinc  protoxide  (ZnO)  67.5, 
water  (H.,0)  7.5  =  100.  Before  the  blowpipe  on  charcoal 
it  yields  the  characteristic  zinc  coating;  further,  a  frag- 
ment ignited  with  cobalt  solution  assumes  a  fine  blue. 
With  hydrochloric  acid  it  forms  a  jelly  (p.  156). 

Smithsonite.     Zinc  carbonate,  ZnCO, 

SMITHSONITE  is  remarkable  because  in  its  common  forms 
looking  so  much  like  calamine.  Like  it  it  may  have  many 
colors— indeed,  the  smithsouite  brought  of  recent  years  from 
the  old  zinc  mines  of  Laurium  in  Greece  is  remarkable  for 
the  beautiful  shades  of  blue,  green,  yellow,  and  red  which  it 
exhibits  in  different  specimens.  It  is  also  called  dry  bone 
by  the  miners.  It  is  related  to  calcite,  siderite,  and  rhodo- 
chrosite,  and  crystallizes  in  similar  rhombohedral  crystals 
(p.  119),  but  they  are  very  rare.  The  common  form  is  that 


238  MINERALS,  AND   HOW   TO   STUDY    THEM. 

of  mammillary  or  botryoidal  masses,  also  stalactitic  shapes. 
The  hardness  is  5,  and  the  specific  gravity  4.3  to  4.45.  The 
luster  is  vitreous. 

The  composition  ZnC03  gives:  Carbon  dioxide  (C02) 
35.2,  zinc  protoxide  (ZnO)  64.8  =  100.  With  acid  it  ef- 
fervesces, as  do  all  the  carbonates.  It  is  infusible  before 
the  blowpipe,  but  when  heated  very  hot  in  the  oxidizing 
flame  after  moistening  with  cobalt  solution  it  takes  a  green 
color  on  cooling. 

CADMIUM  is  a  rare  element  often  associated  with  zinc, 
for  instance  in  sphalerite.  GREENOCKITE  is  cadmium  sul- 
phide. 

ALUMINIUM  or  ALUMINUM. 

ALUMINIUM  is  one  of  the  most  remarkable  of  metals, 
because  while  it  has  great  tenacity  and  is  in  a  high  degree 
sonorous  and  non-oxidizable  in  the  air,  it  has  a  specific 
gravity  of  less  than  calcite,  or  only  2.5.  In  other  words, 
it  is  only  about  one  third  as  dense  as  iron  and  one  fourth 
as  dense  as  silver,  which  it  somewhat  resembles.  Both  as 
the  pure  metal,  because  of  its  low  density,  and  in  alloys, 
for  example  with  copper  as  aluminium  bronze,  because  of 
their  strength  and  other  remarkable  properties,  it  is  highly 
useful.  As  improved  methods  of  obtaining  it  are  devised 
(e.g.,  by  electrolysis)  and  the  price,  once  very  high,  falls,  its 
use  is  being  increased,  and  we  cannot  now  say  to  what  extent 
in  the  future  it  may  supplant  other  metals,  especially  steel. 

Aluminium  does  not  occur  in  the  native  form,  but  it  is 
one  of  the  commonest  of  the  chemical  elements  and  is  an 
important  constituent  of  a  great  many  minerals. 


DESCRIPTION   OF   MINERAL   SPECIES. 


239 


Corundum  is  oxide  of  aluminium;  gibbsite  and  bauxite 
are  hydrated  oxides,  the  latter  occurring  in  large  quantities, 
but  more  or  less  impure;  cryolite  is  a  fluoride  of  alumin- 
ium and  sodium;  kaolin  is  a  silicate  of  aluminium,  and 
the  many  kinds  of  clay  are  related  silicates,  though  usually 
impure;  the  feldspars  are  silicates  of  aluminium  with  po- 
tassium, calcium,  or  sodium;  further,  the  element  enters 
into  the  composition  of  a  considerable  part  of  the  other 
silicates,  as  mica,  the  zeolites,  etc.  The  supply  of  the 
metal  is  now  chiefly  obtained  from  bauxite,  also  from 
gibbsite  and  cryolite. 

Corundum.     Alumina  or  Aluminium  oxide,  A1303. 

CORUNDUM  is,  next  to  diamond,  the  hardest  of  minerals 
and  one  of  great  interest,  Its  clear  blue  varieties  make 
the  sapphire  of  jewelry,  and  the  clear  red  the  highly-prized 
ruby;  while  the  coarse  and  impure  kinds,  when  pulverized, 
are  our  emery.  When  in  distinct  crystals  it  has  a  hex- 
agonal form,  usually  either  that  of  a  prisrn  or  a  tapering 
pyramid  (Figs.  205,  206).  It  is  also  found  in  massive 
forms,  and  these  often  have  a  frac- 
ture nearly  like  a  cube  in  angle. 

The  hardness  is  9,  so  that  it  will 
scratch  any  other  mineral  except 
the  diamond.  The  specific  gravity 
is  4.0,  which  is  high  for  a  non metal- 
lic mineral,  and  remarkably  high 
for  the  oxide  of  a  metal  of  such  low  density.  It  is  not 
often  that  the  oxide  of  a  metal  is  more  dense  than  the 
metal  itself;  this  great  density  is  obviously  connected  with 


205. 


206. 


240  MINERALS,    AND   HOW  TO   STUDY   THEM. 

the  great  hardness  (see  p.  84).  The  luster,  like  that  of 
most  very  hard  minerals,  is  brilliant  and  adamantine, 
though  rather  dull  in  some  massive  kinds.  The  color  is 
gray  to  brown  or  nearly  black  in  many  of  the  common 
varieties,  called  in  part  adamantine  spar;  bright  blue  in 
the  variety  called  the  sapphire;  red  in  the  ruby ;  purple 
in  the  Oriental  amethyst;  *  yellow  in  the  Oriental  topaz. 

Corundum  is  the  sesquioxide  of  aluminium,  A1203.  It  is 
infusible  before  the  blowpipe  and  unattacked  by  acids. 
When  heated  very  hot  it  gives  with  cobalt  nitrate  the 
characteristic  blue  of  alumina.  To  obtain  this,  since  the. 
mineral  is  so  refractory,  it  should  be  pulverized  carefully, 
then  moistened  with  a  drop  of  nitrate  of  cobalt,  so  as  to 
form  a  paste,  and  this  supported  in  the  loop  of  the  plati- 
num wire  and  intensely  heated. 

Common  corundum  occurs  in  Massachusetts  at  Chester, 
New  Jersey,  Pennsylvania,  and  still  more  in  North  Caro- 
lina and  the  adjacent  states  of  South  Carolina  and  Geor- 
gia; gems  are  rare,  but  when  pulverized  and  washed  from 
the  rock  it  is  used  for  emery.  Beautiful  sapphires  have 
been  obtained  in  Ceylon,  and  rubies  in  Siam  and  Burma 
and  other  places  in  the  East  Indies.  Emery  has  been 
extensively  mined  near  Smyrna,  Asia  Minor,  and  at  Naxos 
and  other  of  the  Greek  islands. 

DIASPORE  is  a  rare  oxide  of  aluminium  (Al^O^H^O) 
yielding  about  15  per  cent  of  water  upon  ignition.  It 


*  The  word  Oriental  in  such  cases  was  formerly  much  used;  it 
meant  originally  coming  from  the  East  or  Orient,  ami  from  that,  as 
applied  to  gems,  of  great  value  as  contrasted  with  stones  of  similar 
color  (for  example,  the  common  amethyst)  but  not  so  highly  prised 


DESCRIPTION   OF   MINERAL   SPECIES.  241 

occurs  in  thin  crystals  or  foliated  masses  with  highly  per- 
fect cleavage;  the  luster  on  the  cleavage-face  is  pearly, 
elsewhere  vitreous.  The  color  is  usually  white  or  nearly 
so.  The  hardness  is  G.5  to  7,  and  the  specific  gravity  3.4. 
This  species  is  often  associated  with  corundum,  as  at 
Chester,  Mass.,  in  Pennsylvania  at  Newlin,  and  elsewhere. 

BAUXITE  is  a  hydrated  oxide  of  aluminium  occurring  in 
earthy  masses  resembling  clay;  also  in  concretionary  forms. 
The  color  varies  from  white  to  gray,  yellow,  also  to  brown 
or  red,  especially  in  the  impurer  kinds.  It  is  not  an  at- 
tractive mineral,  but  is  valuable  as  a  source  of  aluminium. 
It  takes  its  name  from  the  principal  locality  at  Baux  (or 
Beaux),  France;  it  is  also  found  in  our  Southern  States. 

GIBBSITE  is  a  hydrated  oxide  of  aluminium,  A1(OH)S  or 
A1,03.3H30.  It  occurs  in  opaque  white  stalactitic  forms 
and  incrustations,  showing  a  radiated  structure.  It  is  often 
found  with  ores  of  iron  and  manganese,  but  not  usually  in 
large  quantities. 

Spinel.     Magnesium  aluminate,  MgAla04. 

SPINEL  is  a  rather  rare  mineral  containing  in  the  typi 
cal  form  magnesia  and  alumina.     It  is  207. 

usually  found  in  octahedrons,  often  in 
twins,  which  are  therefore  called  spinel 
twins  (Fig.  207).  The  hardness  is  8,  or 
as  great  as  that  of  topaz,  and  the  specific 
gravity  3.5  to  4.  The  color  is  sometimes 
pink,  as  in  the  spinel  ruby  or  balas  ruby, 
which  is  not  to  be  confounded  with  the  true  or  Oriental 
ruby.  It  is  also  blue  and  black.  The  ty?  ical  composition 


242  MINERALS,    AND   HOW  TO   STUDY   THEM. 

is  MgAla04  or  MgO.Al.0,,  but  different  kinds  vary  widely 
from  this. 

CHRYSOBERYL  is  another  rare  mineral  containing  beryl- 
lium and  alumina  (BeO.Ala03).  It  is  interesting  because 
very  hard  (H.  =  8.5),  and  in  some  of  its  forms  used  as  a 
gem,  especially  a  grayish-green  kind  (from  Ceylon)  with 
chatoyant  effect,  hence  called  caVs-eye;*  also  in  a  variety 
from  Siberia  named  alexandrite,  which  is  green  as  ordi- 
narily seen,  but  red  by  transmitted  light  (see  Fig.  121,  p. 
58).  The  common  form  has  a  greenish -yellow  color,  a 
little  resembling  beryl,  whence  it  takes  its  name  of  golden 
beryl. 

Cryolite.     Fluoride  of  Aluminium  and  Sodium,  Na3AlF6. 
CRYOLITE  takes  its  name  from  two  Greek  words  (tcpvos, 
which  mean  ice-stone,  and  it  is  so  called  because 
208.  often  found  in  blocks  which  have  some- 

thing of  the  appearance,  as  slightly 
clouded,  of  blocks  of  ice;  it  is  remark- 
able for  its  easy  fusibility. 

It  is  found  in  crystals  having  nearly  the 
angles  of  a  cube,  though  really  monoclinic 
(see  Fig.  208);  it  also  has  cleavages  in  three  directions, 
which,  unless  carefully  examined,  could  be  confounded  with 
cubic  cleavage.  The  hardness  is  2.5,  and  the  specific 
gravity  3.  The  luster  is  vitreous  to  greasy,  and  the  color 
usually  white,  but  sometimes  reddish  or  brownish. 

The  composition,  Na3AlFf ,  which  may  also  be  written 
3NaF.AlFs,  gives:  Fluorine,  54.4,  aluminium  12.8,  sodium 

*  The  same  name  belongs  to  a  less  beautiful  variety  of  quarts' 
giving  a  similar  effect. 


DESCRIPTION   OF   MINERAL  SPECIES.  243 

32.8  =  100.  It  fuses  with  great  ease  even  in  small  frag- 
ments in  the  candle-flame  without  the  blowpipe.  It  gives 
an  intense  yellow  flame  (soda),  and  also  reacts  for  fluorine. 

Cryolite  is  a  rare  mineral,  and  the  only  locality  where  it 
occurs  in  quantity  is  near  Ivigtut  in  Southern  Greenland. 
Here  it  has  been  mined  for  many  years,  because  useful 
both  for  making  soda  salts  and  as  an  ore  of  aluminium  (it 
is  brought  to  Philadelphia  for  this  purpose).  It  is  also 
found  sparingly  in  Colorado. 

THOMSENOLITE  and  PACHNOLITE  are  fluorides  of  alu- 
minium, calcium,  and  sodium,  which  are  related  to  cry- 
olite and  occur  with  it. 

TURQUOIS,  the  beautiful  precious  stone  having  the 
color  of  robin's-egg  blue,  also  bluish  green  in  less  highly 
prized  varieties,  is  a  hydrated  phosphate  of  aluminium, 
containing  also  a  little  copper  phosphate,  which  is  proba- 
bly the  source  of  the  color.  It  occurs  only  in  compact 
massive  forms,  filling  seams  and  cavities  in  a  volcanic  rock. 
The  early  locality  was  in  Persia,  but  of  late  years  a  num- 
ber of  mines  have  been  opened  in  New  Mexico — some  of 
them  were  worked  hundreds  of  years  ago  by  the  Mexicans. 

WAVELLITE  is  another  hydrated  phosphate  of  aluminium. 
It  is  usually  found  in  globular  or  hemispherical  forms 
with  radiating  structure  (see  Fig.  133,  p.  68)  and  a  crys- 
talline surface.  The  hardness  is  3  to  4,  and  the  specific 
gravity  2.3.  The  color  is  white,  varying  to  yellow  or  green. 
There  are  a  number  of  other  related  aluminium  phos- 
phates, but  they  are  too  rare  to  be  included  here,  except 
perhaps  the  azure-blue  LAZULITE  found  in  monoclinic 
crystals  in  Georgia,  also  elsewhere  in  massive  forms. 


244  MINERALS,    AND    HOW   TO   STUDY   THEM. 

AMBLYGONITE  is  a  rare  phosphate  of  aluminium  and 
lithium  containing  fluorine  (AlP04.LiF).  It  usually  oc- 
curs in  white  cleavable  masses  resembling  albite,  but 
easily  distinguished  by  its  fusibility.  It  melts  before  the 
blowpipe  very  readily  (at  2),  giving  a  red  flame  (lithium) 
with  traces  of  green  (phosphorus).  The  hardness  is  6,  and 
the  specific  gravity  3.05.  It  is  found  in  Maine. 

ALUNITE  is  a  sulphate  of  aluminium  occurring  in  rhom- 
bohedral  crystals  looking  a  little  like  cubes.  Another  sul- 
phate is  ALTJMINITE.  The  ALUMS  are  hydrous  sulphates 
of  aluminium  with  potash,  soda,  etc.  There  are  numerous 
such  compounds  among  minerals. 

DAWSONITE,  from  near  Montreal,  is  a  rare  carbonate  of 
aluminium  and  sodium. 

CALCIUM. 

CALCIUM,  whose  oxide  (CaO)  is  the  familiar  substance 
called  lime,  is  a  white  metal  somewhat  resembling  silver  or 
tin.  It  is  obtained  with  difficulty,  for  example  by  elec- 
trolysis (p.  107),  and  is  in  this  form  of  interest  only  to  the 
chemist.  Its  compounds,  however,  are  numerous  and  im- 
portant, and  it  is  indeed  one  of  the  most  widely  distributed 
of  all  the  elements.  The  carbonate  of  calcium  forms  the 
common  mineral  calcite  and  also  the  less  common  arago- 
nite.  Other  very  important  compounds  among  minerals  are: 
the  fluoride,  fluorite  or  fluor  spar;  the  phosphate,  apatite; 
and  the  sulphates,  gypsum  and  anhydrite.  Calcium  is  also 
an  essential  ingredient  of  many  of  the  silicates,  as  in  some 
of  the  feldspars  and  zeolites,  some  varieties  of  pyroxene  and 
garnet,  and  so  on. 


DESCRIPTION   OF  MINERAL   SPECIES. 


245 


Fluorite  or  Fluor  Spar.  Calcium  fluoride,  CaF,. 
Fluorite,  or  Fluor  Spar,  is  one  of  the  most  beautiful  of 
minerals,  occurring  in  cubic  crystals  and  groups  of  crystals 
(see  Fig.  2,  p.  15),  sometimes  very  large  and  of  a  great 
variety  of  colors,  from  colorless  to  green,  yellow,  brown, 
red,  and  purple.  It  is  common  to  find  the  angles  of  one 
cube  projecting  from  the  faces  of  another,  and  as  the  posi- 
tion of  the  crystals  is  then  such  that  if  one  of  them  were 
revolved  180°  about  a  line  joining  opposite  angles  they 
would  be  brought  into  a  parallel  position,  the  group  is 
jailed  a  twin  (Fig.  211,  also  Fig.  116,  p.  57).  The  edges 
of  the  cubes  are  often  beveled  by  a  pair  of  narrow  planes 


209. 


210. 


211. 


(Fig.  209),  and  one,  three,  or  six  little  faces  (Fig.  210)  are 
sometimes  seen  on  the  solid  angles. 

Octahedral  crystals  are  also  found — occasionally  built  up 
of  minute  cubes— and  also  other  forms,  but  the  cubic  habit 
is  so  important  a  character  that  a  non-metallic  mineral  of 
this  form  at  once  suggests  fluorite  to  the  careful  mineral- 
ogist. These  crystals,  and  the  massive  forms  too,  can  often 
be  recognized  by  the  perfect  octahedral  cleavage  which 
makes  it  easy  to  break  off  the  angles  of  the  cubes,  and  from 
a  large  cube  to  form  by  fracture  a  perfect  octahedron. 


246  MINERALS,    AND   HOW   TO   STUDY   THEM. 

Besides  the  crystallized  forms  there  are  others,  not  so 
easy  to  recognize,  which  are  massive.  These  are  often  fi- 
brous or  columnar  in  structure,  and  one  variety  having  the 
colors  arranged  in  bands  is  used  as  an  ornamental  stone; 
this  includes  the  Derbyshire  blue-John.  There  are,  too, 
granular  kinds  and  those  which  are  closely  compact. 

The  hardness  of  fluorite  is  4,  and  its  specific  gravity  3.2. 
The  variety  in  color,  embracing  many  shades  of  green  and 
purple,  yellow  and  rel,  has  already  been  mentioned;  there 
are  also  colorless  kinds.  The  crystals  are  usually  trans- 
parent, and  sometimes  show  on,  or  near,  the  surface  a 
bright  bluish  color  quite  different  from  that  observed 
when  they  are  looked  directly  through.  The  blue  light 
extends  within  the  crystal  if  it  is  placed  in  the  direct  sun- 
light. This  phenomenon  is  called  fluorescence,  and  having 
been  first  observed  with  fluorite  was  named  accordingly 
from  it.  The  name  fluor  spar  is  one  of  the  oldest  in  min- 
eralogy, and  was  given  because  of  the  use  of  this  species  as 
a  flux  in  smelting. 

Fluorite  is  one  of  a  rather  small  group  of  compounds 
called  fluorides;  its  formula  is  calcium  fluoride,  CaFa,  which 
gives  the  percentage  composition:  Fluorine  48.9,  calcium 
51.1  =  100.  Powdered  and  warmed  with  sulphuric  acid 
in  a  lead  or  platinum  crucible  it  gives  off  hydrofluoric  acid, 
and  a  plate  of  glass,  first  covered  with  a  layer  of  wax  and 
then  written  on  by  a  fine  point,  will  have  the  lines  thus 
exposed  etched  by  the  acid.  This  method  of  ornamenting 
glass  or  of  making  marks,  for  example  on  a  thermometer- 
stem,  is  often  used. 

Fluorite  usually  flies  to  pieces  violently  when  heated  be- 


DESCRIPTION    OF   MINERAL   SPECIES.  247 

fore  the  blowpipe;  but  when  pulverized,  as  explained  on 
page  131,  it  can  be  fused  and  yields  the  yellowish-red  flame 
characteristic  of  lime.  Broken  into  small  fragments  and 
heated  in  a  closed  tube,  not  too  hot,  it  phosphoresces,  that 
is,  becomes  self-luminous,  emitting  sometimes  a  yellow 
light,  also,  as  in  the  variety  chlorophane,  a  beautiful  green. 
This  is  best  seen  in  the  dark.  Even  the  blow  of  a  hammer 
is  enough  to  make  a  mass  yield  a  faint  but  beautiful  phos- 
phorescent light  for  hours  after. 

Fluorite  is  a  common  mineral  in  lead  veins,  and  is  then 
said  to  form  the  "  gangue  "  of  the  ore.  It  occurs  in  this 
way  in  Derbyshire  and  Cumberland  in  England,  and  in  the 
Freiberg  mining  region  of  Saxony.  It  is  also  found  in 
cavities  in  limestone,  as  at  St.  Louis.  A  cave  lined  with 
beautiful  sea-green  cubes,  some  of  them  very  large,  was 
opened  at  Macomb,  N.  Y.,  a  few  years  ago.  Besides  the 
use  of  some  colored  varieties  for  vases,  etc.,  the  massive 
kinds  are  employed  as  a  flux  in  smelting  ores  as  already 
stated,  also  in  making  opalescent  glass. 

Calcite.  Calcium  Carbonate  or  Carbonate  of  Lime,  CaCOs. 

CALCITE,  next  to  quartz,  is  the  most  common  of  mineral 
species,  remarkable  for  its  variety  of  form  both  among  the 
crystallized  and  uncrystallized  varieties.  It  crystallizes  in 
rhombohedrons  and  scalenohedrons  of  great  variety  and 
complexity  of  form,  also  in  hexagonal  prisms.  The  funda- 
mental rhombohedron,  Fig.  213,  has  an  angle  between 
two  adjacent  faces  of  105°  (terminal  edge),  and  each  face 
has  plane  angles  of  102°  and  78°.  Parallel  to  the  faces  of 
this  rhombohedron  there  is  very  perfect  cleavage,  so  that  a 


248 


MINERALS,  AND    HOW   TO   STUDY   THEM. 


large  mass  breaks  easily  under  the  blow  of  a  hammer  into 
fragments  all  showing  the  same  form  (see  Fig.  144,  p.  72). 
This  cleavage  is  the  most  important  character  of  the 
crystallized  varieties. 


212. 


213. 


214. 


215. 


There  are  also  other  rhombohedrons,  flattened  ^/r  obtuse 
and  lengthened  or  acute  in  the  vertical  direction,  as  shown 
in  Figs.  212,  214,  215,  220.  The  rhomhohedral  angle  for 


DESCRIPTION   OF   MINERAL   SPECIES.  249 

e  (Fig.  212)  is  135°,  for/ (Fig.  214)  79°,  for  M  (Fig.  215) 
is  66°.  Fig.  216  represents  a  hexagonal  prism,  and  Figs. 
217,  218,  219  the  same  with  the  obtuse  rhombohedron  e  of 
Fig.  212 ;  the  angle  me  is  116^°  over  the  horizontal  edge,  and 
ce  (Fig.  218)  is  153f°.  Fig.  221  shows  the  common  scaleno- 
hedron,  the  angles  for  whose  two  kinds  of  terminal  edges  are 
104°  40'  and  144°  24';  the  angle  for  the  zigzag  edge  is 
133°.  Fig.  222  is  a  similar  scalenohedron  twinned,  and  Fig. 
223  a  combination  of  prism  (m),  rhombohedron  (r),  and 
scalenohedron  (v).  See  also  Figs.  81,  82,  p.  41.  The  variety 
crystallizing  in  scaleriohedral  forms,  or  in  acute  rhombo- 
hedrons,  is  often  called  dog-tooth  spar.  There  are  also 
crystals  with  a  combination  of  faces,  or  highly  modified 
crystals  as  they  are  called,  which  can  only  be  deciphered 
by  one  who  has  a  thorough  knowledge  of  crystallography. 
The  remarkable  experiment  by  which  a  twinning  structure 
may  be  imparted  to  a  cleavage  fragment  is  mentioned  on 
p.  59;  as  there  stated,  twinning  lamellae,  often  of  secondary 
origin,  are  very  common  in  large  rhombohedral  crystals. 

A  clear  cleavage  mass  of  calcite,  such  as  that  brought 
from  Iceland,  is  called  Iceland  spar  and  is  useful  for  opti- 
cal prisms.  This  is  because  of  its  remarkable  double  re- 
fraction, or  power  of  dividing  a  ray  of  light  passing  through 
it  into  two  separate  rays,  so  that  a  line  seen  through  it 
appears  double.  This  phenomenon  has  been  already  de- 
scribed and  illustrated  (Fig.  148,  p.  94). 

Besides  the  crystallized  kinds  there  are  those  which 
have  a  granular  structure,  as  statuary  marble,  and  which 
sparkle  in  the  light  because  of  the  multitude  of  cleavage- 
faces.  Other  kinds  are  fibrous  with  a  silky  luster,  like 


250  MINERALS,  AND   HOW   TO   STUDY   THEM. 

satin  spar;  also  close  and  compact,  as  in  ordinary  marble, 
and  then  of  great  variety  of  color,  red,  yellow,  blue,  black, 
and  largely  used  for  ornamental  purposes.  Some  of  these 
kinds  of  marble  still  contain  shells,  which  come  out  dis- 
tinctly when  polished.  These  shells  are  what  we  have  to 
expect  in  such  cases,  for  most  limestone  has  been  formed 
from  the  material  of  shells,  crinoids,  etc.,  left  by  animals 
whose  remains  have  accumulated  in  large  beds  in  the 
ocean  and  afterward  been  hardened,  crystallized,  and 
elevated  into  the  position  in  which  they  are  now  found. 
A  kind  of  shell  marble  with  beautiful  firelike  reflections 
is  called  lumachelle. 

Stalactites  and  stalagmites  are  varieties  of  calcite  which 
224.  are  formed  in  caverns  in  limestone  rocks. 

The  water,  charged  more  or  less  with  the 
gas  carbon  dioxide,  has  the  power  of 
dissolving  these  rocks  as  it  works  its  way 
through  them  and  the  calcium  carbonate 
in  solution  is  again  slowly  deposited  in  the 
forms  here  described.  The  stalactites 
hang  like  icicles  (Fig.  224)  from  the  roof 
of  the  cavern,  and  the  stalagmites  are 
made  by  the  deposit  from  the  drippings 
on  the  floor  beneath.  They  are  sometimes 
very  large  and  have  often  great  beauty 
and  variety  of  shape;  a  cave  like  the 
Luray  cavern  or  the  Adelberg  grotto  at 
Trieste  is  a  fairyland  of  strange  and 
beautiful  forms.  These  deposits  often  have  a  banded 
structure  and  sometimes  occur  on  a  large  scale,  so  that  the 


DESCRIPTION  OF   MINERAL  SPECIES.  251 

rock  can  be  quarried  and  used  as  an  ornamental  stone. 
The  Mexican  onyx  is  such  a  variety  of  calcite,  a  kind  of 
stalagmite  or  water  deposit,  of  great  delicacy  of  coloring, 
beautifully  translucent  and  used  for  ornamental  purposes 
in  a  great  variety  of  forms.  Other  kinds  of  calcite,  formed 
by  the  deposit  from  waters  containing  carbonate  of  lime, 
are  cole  sinter  or  calc  tufa,  which  often  shows  the  impres- 
sion of  leaves;  agaric  mineral  or  rock-milk,  a  soft  pow- 
dery material;  rock-meal,  a  light  white  cottonli.ke  sub- 
stance. 

Calcite  in  its  normal  crystallized  varieties  has  a  hardness 
of  3,  and  a  specific  gravity  of  2.7.  The  luster  is  usually 
vitreous,  but  silky  in  satin  spar  and  dull  in  some  earthy 
forms.  It  may  be  quite  colorless,  or  pale  yellow,  pink, 
blue,  less  often  dark-colored  except  in  the  marbles,  which 
are  even  jet-black. 

Calcite  is  calcium  carbonate,  or  carbonate  of  lime, 
CaCOs,and  the  percentage  composition  is:  Carbon  di- 
oxide (CO,)  44,  lime  (CaO)  56  =  100.  In  the  case  of 
this  mineral  heat  alone  is  enough  to  separate  it  into  these 
two  parts  and  this  method  is  taken  (as  in  a  lime-kiln)  both 
for  obtaining  the  quicklime  employed  in  making  mortar, 
and  also  carbon  dioxide,  or  carbonic-acid  gas,  used  under 
pressure  for  charging  soda-water  fountains.  Calcite  does 
not  fuse  before  the  blowpipe,  but  a  fragment  after  being 
heated  (notice  that  it  glows  when  ignited)  and  cooling 
turns  a  piece  of  moistened  turmeric-paper  brown  (p.  131). 
In  dilute  hydrochloric  acid  it  effervesces  at  once,  giving  off 
bubbles  of  carbon  dioxide  (carbonic-acid  gas),  and  this  is 
tie  simplest  chemical  test  by  which  to  identify  it. 


252  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Dolomite  (p.  260)  and  siderite  (p.  223),  which  belong  to 
the  "  Calcite  Group"  (see  p.  119),  effervesce  in  acid  also,  but 
only  when  in  the  state  of  a  fine  powder  or  on  being  heated. 

The  occurrence  of  some  of  the  kinds  of  calcite  has 
already  been  spoken  of.  Limestone  rocks  are  found  in 
great  quantities  at  many  points  in  the  world;  in  the 
United  States  they  are  especially  common  through  the 
central  states  of  Illinois,  Iowa,  Wisconsin,  etc.  In  cavities 
in  these  rocks  the  crystallized  forms  occur,  beautiful 
cavities  or  geodes  being  very  common.  It  is  also  found 
with  ores  of  the  metals,  as  of  lead,  copper,  and  silver,  and 
often  in  beautiful  crystals  wonderful  for  their  size  and 
complexity  of  crystallization.  The  mines  of  Lake  Superior 
afford  fine  specimens,  also  those  of  Derbyshire  and  else- 
where in  England  and  the  Harz  region  of  Germany. 

Calcite  in  its  crystalline  varieties  is  easily  recognized  by 
its  cleavage  and  softness,  and  a  drop  of  acid  on  a  frag- 
ment in  a  watch-glass  or  test-tube  effervesces  at  once. 

Calcite  is  useful,  as  already  stated,  for  optical  purposes 
in  the  form  of  Iceland  spar;  also  as  an  ornamental  stone 
and  for  building  purposes  in  the  many  kinds  of  marble. 
Common  limestone  when  burned  yields  the  quicklime 
essential  for  mortar;  further,  the  lime  obtained  from  cer- 
tain kinds,  containing  some  foreign  substances  (as  silica 
and  alumina)  has  the  property  of  becoming  firm  and  solid 
under  water  and  is  hence  called  hydraulic  lime,  the  lime- 
stone being  then  named  hydraulic  limestone. 

Aragonite.     Calcium  carbonate,  CaCO,. 
ARAGONITE  is  also  calcium  carbonate,  having  thus  the 


DESCRIPTION   OF   MINERAL   SPECIES. 


253 


same  composition  as  the  species  calcite;  but  it  crystal- 
lizes in  the  orthorhombic  system,  and  though  the  forms 
may  occasionally  look  like  those  of  calcite,  they  are  easily 
distinguished  because  they  do  not  show  its  cleavage.  The 
crystals  are  commonly  slender  needles  (Fig.  225);  there 
are  also  six-sided  prisms  335.  236. 

which  are  really  com- 
pound or  twin  crystals 
(Fig.  226).  Besides  these 
there  also  occur  other 
kinds,  as  the  delicate 
coral  like  flos-ferri — or 
flower  of  iron — found  in 
iron  mines  (Fig.  227), 
and  further  massive 
kinds. 

Aragonite  is  a  little  harder  than  calcite,  3.5  to  4  instead 
of  3,  and  distinctly  denser,  2.9  instead  of  2.7.     Its  luster 


:.s  vitreous  mostly,  but  on  the  cross-fracture  of  crystals 
plainly  resinous,  which  aids  the  skilled  eye  in  recognizing 


254 


MINERALS,  AND    HOW   TO    STUDY   THEM. 


this  species.  The  composition  of  aragonite  and  its  be- 
havior with  acid  and  before  the  blowpipe  are  the  same  as 
with  calcite. 

Aragonite  is  not  so  common  as  calcite,  but  like  it  is 
a  deposit  from  waters  containing  calcium  carbonate;  the 
question  of  temperature  is  an  important  element  in  deter- 
mining which  of  the  two  species  is  formed. 

Apatite.     Calcium  phosphate,  SCa^Og.CaF,. 

APATITE  can  usually  be  recognized  by  its  hexagonal 
form.  A  common  kind  is  that  occurring  in  long  six-sided 
prisms,  terminated  by  a  low  hexagonal  pyramid  (Figs.  228, 
229);  another  is  in  short,  stout  hexagonal  prisms,  with 
often  a  large  number  of  modifying  planes  (Fig.  230);  and 
still  another  is  like  a  low  pyramid.  The  angle  between 

228.  229.  230. 


two  adjacent  pyramidal  faces  (x)  is  142°  16',  and  that  of 
x  on  m  is  :30°  18',  of  x  on  c  139°  42'.  There  are  also 
massive  forms  showing  no  crystallization. 

The  hardness  is  5,  so  that  it  can  be  scratched,  but  not 
very  easily,  by  the  knife;  its  specific  gravity  is  3.2.  The 
luster  is  usually  vitreous,  but  sometimes  resinous.  The 
common  color  of  the  long  prisms  is  a  dull  green,  but  they 
are  also  yellow,  red,  brown,  black,  and  sometimes  violet; 


DESCRIPTION   OF   MINERAL  SPECIES.  255 

the  smaller  crystals  are  often  clear  and  colorless;  a  variety 
in  clear  yellow-green  crystals  is  called  asparagus-stone. 

Apatite  is  essentially  calcium  phosphate,  Cat(P04)a,  but 
it  also  contains  a  little  fluorine  or,  less  often,  chlorine  (or 
both),  and  the  two  varieties  are  distinguished  as  fluor- 
apatite  and  chlor -apatite.  This  distinction  is  not  a  very 
important  one  and  can  be  made  out  only  by  a  chemical 
analysis.  Apatite  is  difficult  to  fuse  before  the  blowpipe, 
and  on  this  account  it  does  not  give  the  bluish-green  color 
characteristic  of  phosphoric  acid,  unless  first  touched  with 
a  drop  of  sulphuric  acid.  It  is  easily  soluble  in  hydro- 
chloric acid,  also  in  nitric  acid :  and  if  to  this  last  some 
ammonium-molybdate  solution  (in  nitric  acid)  is  added, 
a  bright  yellow  powdery  precipitate  separates  on  gently 
heating;  this  is  a  delicate  test  for  phosphoric  acid. 

Apatite  is  a  rather  widely  disseminated  mineral,  as  in 
granite,  also  in  limestone,  and  with  ores  of  iron  and  tin; 
in  many  igneous  rocks  it  occurs,  as  revealed  by  the  micro- 
scope, in  minute  crystals.  It  is  also  found  in  large  crys- 
tals, sometimes  as  big  as  a  nail-keg,  and  in  masses,  associ- 
ciated  with  pyroxene,  scapolite,  titanite,  also  zircon,  vesu- 
vianite  and  other  species,  in  veins  in  the  crystalline  rocks 
of  Canada  and  Norway;  in  both  countries  it  has  been 
mined  extensively  in  recent  years.  By  treating  with 
sulphuric  acid  soluble  phosphate  is  formed,  which  is  em- 
ployed to  fertilize  the  land.  Phosphorus  is  also  manu- 
factured  from  apatite. 

Related  to  apatite  is  the  phosphate  rock,  which  forms 
extensive  phosphate  deposits  in  South  Carolina  and 
Florida;  this  has  a  great  economic  importance.  Guano, 


256  MINERALS,  AND   HOW  TO   STUDY  THEM. 

as  of  the  West  Indian  islands,  also  consists  largely  of  cal- 
cium phosphate. 

Gypsum.     Calcium  Sulphate  or  Sulphate  of  Lime, 
CaS04  +  211,0. 

GYPSUM  occurs  in  monoclinic  crystals,  and  these  often 
have  the  form  shown  in  Fig.  231;  twin  crystals  are  also 
232.  also  common,  especially  those 

of  the  "swallow-tail"  type, 
like  Fig.  232. 

The  crystals  have  very  per- 
fect cleavage  parallel  to  the 
side  face  (b)  of  Figs.  231 
and  233,  and  sometimes  very 
large  thin  and  perfectly 
transparent  plates  may  be 
obtained. 

The  variety  yielding  these  is  called  selenite  (from  the 
Greek  word,  ae\r)vrj,  meaning  moon}.  These  plates  look 
a  little  like  mica,  but  are  much  softer,  and,  though  somer 
what  flexible,  are  yet  brittle  and  quite  inelastic.  When 
broken  carefully  a  plate  parallel  to  b  (Fig.  234)  shows  also 
two  other  cleavages:  in  the  direction  of  the  front  edge 
(m  or  a)  it  breaks  rather  sharply  with  a  conchoidal  edge, 
while  in  the  direction  of  t  in  the  figure  the  plate  is  some- 
what flexible  and  separates  with  a  fibrous  fracture.  The 
plane  angles  of  this  cleavage  plate,  at,  shown  in  Fig  234, 
are  66°  and  114°.  Further,  the  angle  of  the  front  edge 
mm  of  Fig.  233  is  11H°,  and  of  the  edge  II  is  143|°. 
(jypsum  also  occurs  in  fibrous  forms  called,  like  the 


DESCRIPTION   OF   MINERAL   SPECIES.  *57 

similar  variety  of  calcite,  satin  spar;  it  is  easily  distin- 
guished from  calcite  because  so  much  softer.  There  are, 
further,  foliated  and  stellate  forms  (see  Fig.  134,  p.  68) 
and  those  which  are  granular  and,  2g3  „„, 

again,  earthy,  as  in  impure  massive 
gypsum. 

The  hardness  of  gypsum  is  only  2, 
and  it  is  hence  easily  scratched  by 
the  nail,  though  not,  on  the  other 
hand,  having  the  very  soapy  feel  of 
talc  (whose  hardness  is  1);  the 
specific  gravity  is  2.3.  The  luster  is  pearly  on  the  face 
of  perfect  cleavage,  otherwise  subvitreous;  it  is  also  silky 
in  some  fibrous  forms,  and  in  earthy  kinds  dull.  The 
selenite  variety  is  clear  and  colorless;  the  massive  kinds 
are  generally  snowy  white,  as  in  the  variety  called  ala- 
baster, though  they  may  be  reddish  and,  when  quite  im- 
pure, almost  black. 

Gypsum  is  hydrous  calcium  sulphate,  CaS04  +  2HaO, 
which  gives  the  percentage  composition:  Sulphur  trioxide 
(S0a)  46.6,  lime  (CaO)  32.5,  water  (HaO)  20.9  =  100.  It 
dissolves  quietly  in  hydrochloric  acid.  In  the  forceps  a 
fragment  becomes  opaque  white  and  exfoliates,  fusing  to 
a  globule  which  gives  an  alkaline  reaction  with  turmeric- 
paper.  A  large  amount  of  water  is  given  off  freely  in  tne 
closed  tube,  and  the  residue  is  finally  the  anhydrous  sul- 
phate. This  is  made  on  a  large  scale  and  called  plaster 
of  Paris,  because  so  produced  near  Paris.  This  substance 
has  the  property  of  becoming  very  hard  when  ground 
and  mixed  with  water,  and  is  hence  extensively  used  for 


258  MINERALS,  AND    HOW  TO    STUDY   THEM. 

making  plaster  casts,  for  the  hard  finish  for  walls,  and 
for  other  uses.  Gypsum  is  also  largely  used  when 
ground  up  for  improving  soils.  The  variety  alabaster 
is  soft  and  easily  cut  into  vases  and  other  ornamental 
objects. 

Gypsum  is  a  very  common  mineral,  forming  extensive 
beds  in  regions  of  limestone,  also  present  in  clay  beds  and 
in  connection  with  deposits  of  rock-salt  and  salt-wells. 
It  is  found  in  curious  forms  in  the  Mammoth  Cave, 
Kentucky. 

ANHYDRITE  receives  its  name  because,  while  consisting 
of  calcium  sulphate  (CaSOJ,  it  does  not,  like  gypsum,  con- 
tain water.  It  is  rarely  found  in  crystals,  more  commonly 
in  forms  with  what  looks  like  cubic  cleavage,  but  careful 
examination  shows  that  the  three  cleavages  are' unlike,  and 
optical  examination  confirms  this  and  proves  the  species  to 
belong  to  the  orthorhombic  system.  There  are  also  com- 
pact kinds.  The  hardness  is  3  to  3.5;  specific  gravity 
about  3;  the  luster  vitreous  to  pearly;  color  white  to  gray 
or  bluish,  also  red.  It  is  not  a  common  mineral,  and  is 
usually  associated  with  rock-salt  and  gypsum. 

SCHEELITE  is  a  mineral  consisting  of  calcium  tungstate 
(CaW04).  Like  other  tungsten  compounds  it  has  a  higli 
specific  gravity  (G.  =  6),  in  fact  there  is  perhaps  no  other 
mineral,  outside  of  the  compounds  of  lead,  which  like  this 
is  white  to  pale  yellow  in  color,  or  even  colorless,  and  at 
the  same  time  has  so  high  a  density.  It  is  found  in 
tetragonal  crystals  or  massive  forms,  and  is  often  asso- 
ciated with  cassiterite  (tin-stone),  topaz,  fluorite,  and  other 
species. 


DESCRIPTION   OF   MINERAL   SPECIES.  XM 

It  is  interesting  to  note  here  that  some  eom^ounds  of 
tungsten  are  used  for  pigments;  sodium  tungstate  has 
the  effect  of  making  cloths  soaked  in  it  entirely  incom- 
bustible. Tungsten  is  also  employed  in  certain  rare 
alloys. 

Of  the  many  other  natural  compounds  of  calcium,  the 
following  may  be  mentioned. 

PHARMACOLITE  is  a  hydrous  arsenate  occurring  in  white 
monoclinic  crystals,  more  commonly  in  silky  fibers  or 
botryoidal  forms.  The  color  is  usually  white  or  grayish. 

COLEMANITE  is  calcium  borate,  occurring  in  fine  mono- 
clinic  crystals,  colorless  or  milky  white,  in  San  Bernardino 
County,  California.  ULEXITE  is  a  borate  containing  both 
calcium  and  sodium. 

PEROVSKITE  is  calcium  titanate,  a  rare  mineral  occurring 
in  isometric  crystals.  MICROLITE  is  essentially  calcium 
tantalate;  it  is  mentioned  again  on  a  later  page  (p.  271). 

MAGNESIUM. 

MAGNESIUM  is  the  metal  which  is  present  in  the  oxide 
(MgO)  called  magnesia.  It  is  a  white  metal  resembling 
calcium  and  closely  related  to  it  chemically.  In  the  form 
of  a  thin  strip  or  ribbon  it  burns  readily  in  the  air,  yielding 
a  very  brilliant  white  light,  which  is  often  used  as  a  source 
of  illumination  in  photography.  The  chemical  product  of 
the  combustion  is  the  white  oxide  just  spoken  of.  There 
are  a  number  of  prominent  compounds  of  magnesium  oc- 
curring among  minerals,  and  it  enters  into  the  composition 
of  many  of  the  silicates. 


MINERALS,  AND   HOW   TO    STUDY   THEM. 

Brucite.     Magnesium  hydrate,  Mg(OH)2. 

BRUCITE  is  found  in  white  or  greenish  scales  or  plates, 
with  perfect  cleavage,  often  transparent.  These  are  hex- 
agonal in  outline,  but  they  may  be  united  together  in  ro- 
settelike  groups,  and  then  the  form  is  not  distinct.  There 
is  also  a  fibrous  variety  called  nemalite. 

The  hardness  is  only  2.5,  so  that  the  plates  are  about 
as  hard  as  mica,  which  they  somewhat  resemble;  on  the 
other  hand,  they  are  a  little  harder  than  gypsum,  and  much 
more  so  than  talc.  The  luster  is  distinctly  pearly,  and  the 
color  often  slightly  greenish. 

Brucite  is  the  hydrate  or  hydrated  oxide  of  magnesium, 
Mg(OII)2  or  MgO.H20.  In  the  closed  tube  it  yields  water, 
and  a  fragment  heated  in  the  forceps  turns  opaque  from 
loss  of  water,  and  after  being  moistened  with  cobalt  solu- 
tion and  again  ignited  it  assumes  a  pale  pink  color — the 
reaction  for  magnesia. 

MAGNESITE,  the  carbonate  of  magnesium,  MgC03,  is  not 
a  common  mineral.  It  is  found,  but  only  rarely,  in  crys- 
tals and  cleavable  forms,  which  are  interesting  because  re- 
sembling some  of  those  of  calcite,  but  the  ordinary  form  is 
that  of  a  white  compact  mass  looking  like  chalk,  but 
harder. 

Dolomite.     Calcium-magnesium  carbonate,  CaMg(C03).,. 

DOLOMITE,  the  carbonate  intermediate  between  calcite 
and  magnesite,  is  a  common  and  important  mineral.  It 
has  the  same  form  as  calcite,  and  often  cannot  certainly 
be  distinguished  from  it  by  simple  inspection.  The  crys- 


DESCRIPTION   OF   MINERAL   SPECIES.  261 

tals  have  one  peculiarity,  however,  in  that  the  rhombo- 
hedral  faces  are  almost  always  curved,  giving  a  convex 
surface.  This  is  not  true  of  calcite,  but  is  the  case 
with  rhombohedrons  of  siderite,  the  carbonate  of  iron;  so 
that  the  figure  (Fig.  235)  given 
of  that  species  is  repeated  here. 
Sometimes  the  crystals  are  formed 
of  many  smaller  crystal?,  and  all 
so  much  curved  as  to  give  a  saddle- 
shaped  form,  as  illustrated  in  Fig.  236.  The  white  cleavage- 
masses  of  dolomite  also  sometimes  show  slightly  curved 
surfaces,  but  this  is  not  universally  true,  and  the  granular 
forms  look  like  granular  calcite. 

Dolomite  is  a  little  harder  than  calcite,  H.  =  3.5  to  4, 
instead  of  3,  and  a  little  denser,  G.  =  2.8  to  2.9.  It  also 
differs  in  being  less  easily  attacked  by  acid;  thus  a  frag- 
ment of  calcite  dropped  into  cold  dilute  hydrochloric  acid 
effervesces  strongly  at  once,  while  a  similar  piece  of  pure 
dolomite  is  hardly  attacked  at  all,  and  the  effervescence 
does  not  start  up  unless  the  acid  is  warmed  or  the  mineral 
pulverized.  The  luster  is  vitreous,  though  pearly  in  some 
kinds;  it  varies  from  colorless  to  white  and  pale  green, 
pink,  and  brown;  it  is  transparent  to  translucent. 

Normal  dolomite,  as  it  is  called,  or  the  common  crystal- 
lized dolomite,  consists  chemically  of  the  carbonates  of  cal- 
cium and  magnesium  in  equal  proportions,  CaMgCa06  or 
CaCO,.MgCOs.  In  other  kinds,  however,  the  ratio  of  the 
two  carbonates  may  vary  somewhat  widely.  There  are 
also  other  related  minerals  (as  ANKERITE)  which  contain 
iron  as  well  as  calcium  and  magnesium,  and  others  which 


262  MINERALS,  AND    HOW   TO   STUDY   THEM. 

contain   magnesium  and  iron  only  (as  BREUNERITE  and 
MESITITE).     Much  white  marble  is  dolomite,  not  calcite. 

BORACITE  is  a  rare  borate  of  magnesium  containing 
chlorine.  It  occurs  in  glassy  crystals  which  are  tetra- 
hedral  like  Figs.  31  to  35,  p.  29;  color  also  white  to  yellow 
or  gray.  Hardness  =  7;  specific  gravity  2.9  to  3;  luster 
vitreous,  inclining  to  adamantine.  In  the  forceps  it  fuses 
easily,  yielding  a  green  flame  (boron)  and  turning  pink 
with  cobalt  solution  (magnesium,  see  p.  260). 

EPSOMITE  is  hydrated  magnesium  sulphate,  occurring 
sparingly  as  a  mineral,  for  example  in  some  caves.  The 
artificial  compound  is  commonly  employed  in  medicine. 

Magnesium  is  also  present  in  a  great  many  silicates,  as 
talc,  serpentine,  sepiolite  or  meerschaum,  also  the  varieties 
of  pyroxene  and  amphibole;  further  staurolite,  some  tour- 
maline, etc. 

BARIUM. 

BARIUM  is  a  metal  only  known  in  the  laboratory,  where 
it  can  be  obtained  from  some  of  its  compounds.  It  is  a 
heavy  metal,  and  takes  its  name  from  this  fact  from  the 
Greek  word  for  heavy  (fiapvs\.  All  of  its  salts  have  also 
high  density. 

The  chemist  can  make  a  large  variety  of  compounds  of 
barium,  but  the  only  prominent  minerals  in  which  it 
occurs  are  the  sulphate,  barite,  and  the  carbonates,  with- 
erite  an  barytocalcite.  It  also  appears  in  a  few  silicates 
(as  the  zeolite,  harmotome),  but  they  are  all  rare  minerals. 

Barite,  or  Heavy  Spar.     Barium  sulphate,  BaS04. 
BARITE,  the  sulphate  of  barium,  is  first  of  all  character- 


DESCRIPTION   OF  MINERAL   SPECIES. 


263 


ized  by  its  high  density,  and  on  this  account  it  is  often 
called  heavy  spar.  Though  looking  very  like  calcite  in 
some  varieties  and  feldspar  in  others,  it  is  at  once  recog- 
nized even  in  a  small  specimen  by  a  trained  hand,  for  its 
specific  gravity  is  4.5,  while  that  of  the  others  is  only  2.7. 
Barite  is  often  found  in  beautiful  clear  crystals,  usually 
rhombic  prisms  or  flat  tabular  forms.  The  last  are  ofteq 
united  in  diverging  groups  like  the  leaves  of  a  partly^ 
opened  book.  Some  of  the  common  forms  are  shown  in 
the  accompanying  figures;  it  will  be  seen  that  the  "  habit " 
varies  widely.  In  Figs.  237, 238, 239, 245  the  form  is  tabular, 


237. 


parallel  to  the  basal  plane  (of  perfect  cleavage) ;  in  Figs.  237, 
238  the  fundamental  or  cleavage  prism  (m)  is  developed 
and  crystals  occur  which  are  elongated  in  this,  the  vertical, 
direction;  in  Figs.  240  to  244  the  habit  is  prismatic,  but 
for  Figs.  240,  242,  244  in  the  direction  of  one  lateral 
axis  and  for  Figs.  241,  243  in  that  of  the  other.  It  will  be 
understood  that  Figs.  240,  242  show  the  same  form,  and 


264  MINERALS,  AND   HOW  'TO   STUDY  THEM. 

also  Figs.  241  and  243,  but  in  different  positions,  the  second 
in  each  case  corresponding  to  that  of  Figs.  237,  238. 

The  angle  of  the  prism  m  (front  edge)  is  10H°;  the 
angles  of  d  and  o  (side  edges,  Fig.  239)  are  77f°  and  105£°; 
while  the  angle  cd  is  141°  and  co  is  127^°. 

Barite,  as  already  intimated,  has  cleavage  parallel  to 
three  faces,  two  of  these  (in)  making  with  each  other 
angles  of  about  101^°  and  78^°  and  the  other  (c)  at  right 
angles  to  each  of  these;  parallel  to  the  last  cleavage  face 
(c)  the  crystals  often  show  pearly  luster.  The  form 
yielded  by  the  cleavage  is  shown  in  Fig.  237.  Barite  is 
also  found  in  cleavable  masses,  and  in  lamellar  to  granular 
compact  and  earthy  forms  which  show  no  cleavage.  The 
last  often  have  a  variety  of  colors  and  make  a  handsome 
marble  when  polished ;  banded  kinds  resembling  stalagmite 
occur.  There  are  also  granular  forms  looking  like  statuary 
marble,  and  fibrous  kinds.  The  hardness  is  2.5  to  3.5,  and 
the  specific  gravity  4.3  to  4.6.  The  luster  is  usually  vit- 
reous, though  often  pearly  on  c  as  above  stated. 

The  formula  is  BaS04,  which  gives  the  percentage  com- 
position: Sulphur  trioxide  (SOS)  34.3,  baryta  (BaO)  65.7 
=  100.  Barite  fuses  (see  p.  131)  in  the  forceps  rather 
easily  and  gives  the  pale  yellowish-green  flame  character- 
istic of  barium;  the  fused  mass  when  (after  cooling)  it  is 
placed  on  a  piece  of  moistened  turmeric-paper  colors  it 
reddish  brown.  Fused  with  sodium  carbonate  on  char- 
coal it  reacts  for  sulphur  (p.  146).  It  is  insoluble  in  acids. 

Barite  is  a  common  mineral,  especially  in  certain  mining 
regions,  as  those  of  England,  in  Cornwall,  Cumberland, 
Derbyshire;  also  in  Saxony,  Hungary,  and  in  Colorado. 


DESCRIPTION   OF  MINERAL   SPECIES.  265 

It  is  often  the  gangue  mineral  of  lead,  copper,  and  other 
ores.  It  also  forms  veins  in  certain  rocks,  as  sandstone  at 
Cheshire,  Conn.  It  is  used  extensively  as  a  pigment,  or 
rather  an  adulterant  of  white  lead ;  also  to  glaze  paper. 
Barium  salts  are  also  used  for  the  colored  fire  at  the 
theater. 

Witherite.     Barium  carbonate,  BaCO,. 

WITHERITE,  the  carbonate  of  barium,  is  a  much  less 
common  mineral  than  barite,  but  like  it  it  has  a  high  spe- 
cific gravity.  It  sometimes  occurs  in  crystals,  which  may 
have  the  form  of  six-sided  pyramids,  a  little  suggesting 
quartz  in  aspect  (Fig.  246);  these  crystals,  however,  are 
complex  forms  due  to  the  combination 
or  twinning  of  several  crystals.  The 
more  common  form  of  witherite  is 
that  in  fibrous  masses.  The  hardness 
is  about  3.5,  and  the  specific  gravity 
4.3.  The  luster  is  vitreous,  inclining 
to  resinous  on  fracture  surfaces,  and 
the  color  white,  pale  yellowish  or 
grayish. 

The  composition,  BaC03,  gives:  Carbon  dioxide  (C0a) 
22.3,  baryta  (BaO)  77.7  =  100.  It  fuses  easily  in  the 
forceps  and  gives  a  yellow-green  flame.  In  hydrochloric 
acid  it  dissolves  with  effervescence,  the  solution  yielding  a 
heavy  white  precipitate  (barium  sulphate)  if  a  little  sul- 
phuric acid  is  added.  Witherite  is  used  in  the  refining  of 
sugar. 

BARYTO-CALCITE  is  a  rather  rare  carbonate  of  barium 


266  MINERALS,  AND   HOW   TO    STUDY    THEM. 

and  calcium  occurring  in  white  or  yellowish   monoclinio 
crystals;  also  in  massive  forms. 

STRONTIUM. 

STRONTIUM  is  the  metal  which  is  present  in  the  vari- 
ous salts  characterized  by  the  beautiful  red  color  which 
they  give  to  the  flame.  The  nitrate  is  thus  used  in  fire- 
works and  red  fire;  the  hydrate  is  used  for  preparing  and 
refining  beet-sugar  and  in  extracting  crystallized  sugar 
from  molasses.  The  common  minerals  containing  stron- 
tium are  the  sulphate,  celestite,  and  the  carbonate,  stron- 
tianite ;  it  is  also  present  in  a  few  silicates. 

Celestite.  Strontium  sulphate,  SrS04. 
CELESTITE  is  a  mineral  closely  related  to  barite  in  ap- 
pearance, in  fact  so  closely  related  that  they  often  cannot 
be  certainly  told  apart  without  a  blowpipe  test  to  show  the 
color  of  the  flame.  It  is  found  in  crystals  which,  like 
those  of  barite,  are  sometimes  flat  tables  (Fig.  247),  some- 
times prisms,  and  the  latter  often  terminated  by  two  pairs 
of  planes  d  and  m  (Fig.  248). 

347.  248. 


The  angles  are  near  those  of  the  barium  sulphate,  barite, 
the  two  species  being  isomorphous  as  defined  on  p.  119. 
The  prismatic  angle  m  (front  edge)  is  104°,  also  the  angles 
of  d  and  o  (side  edge)  are  79°  and  104°,  further  the  angles 
cd  =  140i°,  co  =  128°,  cl  =  157i°. 


DESCRIPTION   OF   MINERAL   SPECIES.  267 

The  cleavage  form  is  like  that  of  barite;  that  is,  the  crys- 
tal cleaves  parallel  to  c  and  m.  There  are  also  massive 
cleavable  forms,  and  others  which  are  finely  fibrous.  The 
hardness  of  celestite  is  3  to  3.5,  and  the  specific  gravity 
3.95  to  4.  The  luster  is  vitreous,  or  pearly  on  the  face  of 
cleavage.  The  color  is  commonly  white,  but  the  crystals 
often  show  a  tinge  of  blue,  and  the  same  is  true  of  the 
fibrous  forms,  and  although  this  is  not  an  essential  charac- 
ter, it  is  so  common  that  it  has  given  the  name  to  the 
species  from  the  Latin  ccelestis.  There  is  also  a  red 
variety. 

The  composition  of  celestite,  SrS04 ,  gives :  Sulphur  tri- 
oxide  43.6,  strontia  56.4  =  100.  A  fragment  heated  in  the 
forceps  tinges  the  flame  a  deep  red  and  fuses  at  3  to  a 
white  bead  which  reacts  alkaline  on  turmeric-paper.  It 
is  insoluble  in  acids. 

Celestite  is  often  associated  with  limestone,  and  with 
gypsum,  rock-salt,  and  clay;  it  also  occurs  in  sandstone; 
occasionally  with  metallic  minerals  as  galena  and  sphaler- 
ite. It  is  not  so  common  a  mineral  as  barite. 

Strontianite.     Strontium  carbonate,  SrCOs. 

STRONTIANITE,  the  carbonate  of  strontium,  is  the  other 
important  strontium  mineral,  though  less  common  than 
celestite.  It  is  sometimes  found  in  prismatic  crystals, 
occasionally  transparent,  but  commonly  in  fibrous  or 
granular  masses  of  a  white,  pinkish,  or  greenish  color. 
The  hardness  is  3.5  to  4,  and  the  specific  gravity  about  3.7. 
The  luster  is  vitreous,  inclining  to  resinous  on  faces  of  frac- 
ture. 


268  MINERALS,  AND   HOW  TO   STUDY  THEM. 

The  composition  SrCOs  gives:  Carbon  dioxide  (C0a)  29.9, 
strontia  (SrO)  70.1  =  100.  Strontianite  gives  the  same  red 
color  to  the  blowpipe  flame  as  celestite,  but  it  is  at  once 
distinguished  by  its  effervescing  in  hydrochloric  acid. 

Sodium  and  Potassium. 

The  metals  SODIUM  and  POTASSIUM,  though  obtained 
with  some  difficulty,  are  interesting  to  the  chemist  because 
ihey  combine  so  eagerly  with  oxygen.  For  this  reason 
they  can  be  preserved  only  in  some  non-oxidizable  medium, 
as  oil,  and  a  fragment  of  potassium  placed  in  water  takes 
fire  and  burns,  uniting  with  the  oxygen  and  liberating 
hydrogen;  sodium  also  decomposes  water.  The  rare  metals 
caesium  and  rubidium  have  a  similar  chemical  character, 
and  finally  lithium,  the  lightest  of  all  the  metals,  with  a 
specific  gravity  of  only  half  that  of  water. 

Only  a  few  simple  compounds  of  sodium  and  potassium 
are  known,  but  they  both  enter  into  the  composition  of 
very  many  of  the  complex  silicates. 

Of  the  other  metals  named  above,  csesium  and  rubidium 
have  a  very  limited  occurrence,  being  known  only  in  cer- 
tain rare  silicates;  lithium,  however,  enters  into  the  com- 
position of  lepidolite  or  lithia  mica,  rubellite  or  lithia 
tourmaline,  also  the  rare  phosphates  triphylite  (and  related 
compounds),  amblvgonite,  and  a  few  other  species. 

Halite,  or  Rock-salt.     Sodium  chloride,  NaCl. 
HALITE,  or  rock-salt,  is  one  of  the  few  important  min- 
erals which  is  readily  soluble  in  water  and  hence  gives  a 
decided  taste.  This  taste  is  familiar  to  every  one,  for  halite 
is  simply  the  natural  form  of  table-salt. 


DESCRIPTION   OF   MINERAL   SPECIES.  269 

It  crystallizes  in  fine  clear  cubic  crystals  with  perfect 
cubic  cleavage  ;  it  is  also  found  in  granular  cleavable 
masses.  Sometimes  the  crystals  have  the  skeleton  or  hop- 
per shape  spoken  of  on  p.  54. 

The  hardness  is  2.5,  and  the  specific  gravity  2.1.  It  has 
a  vitreous  luster,  and  it  is  colorless  when  perfectly  pure, 
but  from  white  it  passes  through  various  shades  of  red 
and  yellow  ;  occasional  patches  of  a  fine  deep  blue  are  seen 
in  the  clear  crystals. 

The  composition  sodium  chloride,  NaCl,  gives:  Chlorine 
39.4,  sodium  60.6  =  100.  The  deep  yellow  color  given  to 
the  blowpipe  flame  (best  when  fused  on  a  platinum  \\ire) 
is  a  very  characteristic  reaction.  It  also  gives  the  reaction 
for  chlorine  mentioned  on  p.  133. 

Beds  of  salt  are  common  in  many  parts  of  the  world, 
where  it  is  mined  in  large  quantities  for  commercial  use. 
There  are  also  salt-wells  in  certain  regions,  as  in  New  York, 
Ohio  and  other  States,  the  brine  from  which  is  pumped 
up  and  evaporated  for  the  sake  of  the  salt  which  it  yields. 
Salt  is  also  present  in  sea- water,  being  the  most  important 
of  the  saline  substances  in  solution  to  which  the  peculiar 
taste  is  due,  some  of  the  others  being  magnesium  sulphate 
and  chloride.  The  ocean  is  hence  an  important  source  of 
the  salt  of  commerce.  Sodium  chloride  is  also  present 
still  more  largely  in  the  water  of  some  inland  seas,  as  the 
Great  Salt  Lake  in  Utah,  the  Dead  and  Caspian  seas, 
and  many  others.  It  is  to  the  gradual  evaporation  of 
bodies  of  salt  water  that  we  must  look  for  the  explana- 
tion of  the  beds  of  salt  the  occurrence  of  which  has  beer 
alluded  to. 


270  MINERALS,  AND   HOW   TO   STUDY   THEM. 

BORAX,  the  sodium  borate  which  is  so  familiar  to  one 
who  works  with  the  blowpipe  (see  p.  125),  also  occurs  in 
nature,  and  on  so  large  a  scale  in  certain  limited  regions  as 
to  be  extensively  mined.  It  is  found  in  monoclinic  crys- 
tals, often  of  large  size;  they  are  clear  at  first,  but  lose 
water  on  exposure  and  become  white  and  opaque.  Borax 
Lake  in  Lake  County,  California,  as  also  another  lake  of 
the  same  name  in  San  Bernardino  County,  affords  this  min- 
eral; there  are  also  large  deposits  in  Tibet.  Borax  finds 
many  uses  in  the  arts,  as  in  making  glass  and  soap,  in 
soldering,  in  medicine,  etc. 

Some  other  sodium  compounds  occurring  in  nature  are 
the  carbonates,  including  natron  and  trona;  the  fluoride, 
cryolite,  already  described  on  p.  242;  the  nitrate,  called 
soda-niter;  the  sulphates,  thenardite  and  mirabilite  (glauber 
salt),  further,  glauberite  (containing  also  lime).  Sodium 
is  also  present  in  many  silicates,  as  albite,  natrolite,  etc. 

SYLVITE,  or  the  chloride  of  potassium  (KC1),  is  a  rare 
mineral  occasionally  found  associated  with  halite  or  rock- 
salt,  and  closely  resembling  it  in  form,  taste,  and  other 
characters.  It  is  easily  distinguished  by  the  violet  color 
which  it  gives  the  blowpipe  flame. 

Potassium  also  forms  a  number  of  compounds,  as  sul- 
phates, borates,  etc.,  but  they  are  mostly  rare  and  hardly 
to  be  found  in  the  cabinet  of  most  mineralogists.  It  also 
enters  into  the  composition  of  many  silicates,  as,  for 
example,  orthoclase  or  potash  feldspar,  also  muscovite  or 
common  potash  mica.  So,  too,  apophyllite  and  many 
others. 


DESCRIPTION   OF   MINERAL   SPECIES.  271 

A  few  minerals,  interesting  because  they  contain  rare 
elements,  may  be  mentioned  briefly  in  this  place. 

MONAZITE  is  the  most  important  of  these.  It  is  essen- 
tially a  phosphate  of  cerium  and  the  allied  elements  lan- 
thanum and  didyminm,  but  contains  also  varying  quan- 
tities of  thorium.  It  is  to  this  last  element  that  it  owes 
its  importance,  since  the  oxide,  thoria  or  thorina,  is  used 
in  the  preparation  of  the  common  form  of  gas-burner,  in 
which  the  light  is  given  by  the  incandescence  of  a  delicate 
hood  suspended  in  the  flame.  Monazite  was  so  named 
because  of  its  rarity  (from  [jiovd^eiv,  to  be  solitary), 
being  discovered  in  small  isolated  monoclinic  crystals  im- 
bedded in  certain  rocks,  as  gneiss.  It  has,  however,  since 
been  found  in  large  quantities  as  a  rock  constituent,  and 
where,  as,  for  example,  in  North  Carolina,  the  rock  has 
been  disintegrated  by  the  weather,  it  is  possible  to  separate 
it  and  obtain  it  in  large  quantities.  The  crystals  have 
sometimes  a  brilliant  luster,  and  vary  in  color  from  red- 
dish to  yellowish  brown;  hardness  5  to  5.5;  specific  gravity 
5  to  5.2.  It  is  also  found  in  Norway,  Russia,  and  Brazil; 
turnerite  is  a  variety  found  in  Switzerland. 

XENOTIME  is  a  phosphate  of  yttrium  and  other  rare  ele- 
ments; it  occurs  in  tetragonal  crystals  resembling  those  of 
zircon  in  form  and  angles  (see  p.  312).  It  occurs  sparingly 
in  granitic  and  gneissoid  rocks. 

PYROCHLORE  is  a  rare  mineral  containing  niobium,  tho- 
rium, titanium,  also,  as  bases,  calcium,  cerium,  sodium;  it 
is  found  in  octahedral  crystals  of  a  brown  color  in  Norway 
and  in  the  Ural. 

MICRO-LITE,  related  to  pyrochlore,  is  essentially  a  calcium 


272  MINERALS,  AND   HOW   TO   STUDY   THEM. 

tantalate.  It  occurs  in  octahedral  crystals,  usually  very 
small  (whence  the  name),  and  often  imbedded  in  albite, 
as  at  Chesterfield,  Mass.;  also  in  larger  complex  forms 
(Fig.  30,  p.  28),  as  in  Amelia  County,  Virginia. 

SILICON. 

SILICON  is,  next  to  oxygen,  the  most  widely  distributed 
of  the  chemical  elements;  in  fact  it  is  estimated  to  make 
up  about  one  fourth  of  the  earth's  crust.  The  element 
itself  is  known  only  to  the  chemist,  who  obtains  it  with 
some  difficulty  from  its  compounds;  one  form  is  that  of 
reddish  crystals,  resembling  those  of  the  diamond  and 
almost  as  hard. 

The  common  compound  of  silicon  in  nature  is  the  oxide, 
SiO,,  called  usually  simply  silica.  The  well-known  min- 
eral quartz  has  this  composition.  Opal  is  another  kind  of 
silica,  amorphous  and  containing  some  water.  There  is 
also  a  rare  form  found  in  some  volcanic  rocks  in  thin 
transparent  hexagonal  plates;  this  is  the  mineral  TRIDY- 

MITE. 

Silica  is  present  as  the  acid  part  of  a  large  family  of 
compounds,  called  SILICATES,  which  includes  not  only  a 
greater  number  of  minerals  than  any  other  family,  but  also 
some  of  the  commonest  and  most  beautiful  of  species. 
The  feldspars,  micas,  zeolites,  also  garnet,  beryl,  amphibole 
or  hornblende,  pyroxene,  and  topaz  are  all  silicates,  and 
there  are  many  others  which  will  be  mentioned  in  the  pages 
which  follow. 


DESCRIPTION   OF  MINERAL  SPECIES.  273 

Quartz.     Silicon  dioxide,  SiO,. 

QUARTZ,  the  oxide  of  silicon,  is  the  commonest  of  min- 
erals, and  in  some  of  its  varieties  one  of  the  most  beauti- 
ful. It  makes  up  most  of  the  sand  of  the  seashore;  it 
occurs  as  a  rock  in  the  forms  of  sandstone  and  quartzite, 
and  is  a  prominent  part  of  many  other  important  rocks,  as 
granite  and  gneiss.  It  is  a  mineral  which  can  be  usually 
recognized  by  its  form  when  crystallized;  also  by  its  hard- 

249.  251.  252.  253. 


250. 


ness,  conchoidal  fracture,  its  glassy  luster  and  infusibility. 
There  are  so  many  varieties,  however,  that  it  is  only  after 
long  practice  that  one  can  be  sure  of  always  identifying  it 
at  once. 

The  common  form  of  quartz  crystals  is  that  of  a  hex- 
agonal prism  terminated  by  six  pyramidal  planes  each  hav- 
ing the  shape  of  an  acute  isosceles  triangle  (Figs.  249  and 
254).  Sometimes  the  prism  is  entirely  wanting,  and  then 


274 


MINERALS,  AND    HOW   TO   STUDY   THEM. 


the  form  is  like  that  of  Figs.  250  and  255,  which  is  a 
double  hexagonal  pyramid,  or  quartzoid.  It  is  not  uncom- 
mon to  find  three  of  these  pyramidal  faces — every  other  one, 
those  lettered  r  in  Fig.  254 — much  larger  than  the  others; 
or  they  may  be  present  alone  (Figs.  252,  253,  also  256  to 
259).  It  is  because  of  this  fact  and  for  other  reasons  that 
it  is  concluded  that  these  three  planes,  lettered  r,  with 
those  corresponding  to  them  at  the  other  end  of  the  crys- 


254. 


255. 


256. 


257. 


tal,  belong  together  and  are  different  from  the  subordinate 
set  (lettered  z).  These  six  planes  (r)  form  a  rhombo- 
hedron  not  very  far  from  a  cube  in  angle  (see  Fig.  257), 
and  hence  the  fundamental  form  is  said  to  be  rhombo- 
hedral  instead  of  hexagonal. 

Further,  when  the  smaller  modifying  planes  on  a  quartz 
crystal  are  studied — and  the  crystals  are  sometimes  very 
complex — or  when  the  molecular  structure  is  investigated 


DESCRIPTION   OF   MINERAL   SPECIES.  275 

by  the  etching  process  already  described  (see  p.  64  and 
Fig.  132),  or  by  the  method  of  pyro-electricity  (p.  97),  it  is 
found  that  this  structure  is  highly  intricate.  This  is  a 
matter,  indeed,  for  the  skilled  mineralogist,  but  even  a  be- 
ginner may  learn  to  recognize,  for  example,  on  a  Swiss 
crystal  of  smoky  quartz  the  difference  between  the  forms 
shown  in  Figs.  258  and  259.  The  first,  which  is  called  a 
right  handed  crystal,  has  the  little  planes,  like  that  lettered 
x,  to  the  right  above  the  prismatic  face  m,  which  is  below 
the  face  r  (usually  identified  by  being  larger  than  z); 
while  the  other  is  a  left-handed  crystal  and  has  such  a 
plane  to  the  left  above  m.  With  this  difference  goes  also  a 
more  important  distinction  in  optical  character  which  it  is 
for  the  advanced  student  in  mineralogy  to  learn  about. 
The  important  angles  between  the  faces  are  as  follows: 

rr'  =    94°  14'  and  85°  46'  (Figs.  256,  257); 
rz   =  133°  44'   (over  the  terminal  edge), 

103°  34'  (horizontal  edge); 
mr  =  141°  47'  (horizontal  edge,  Fig.  254); 
ms  =  142°    2'  (Fig.  258). 

The  difficulty  in  deciphering  even  simple  crystals  of 
quartz  is  much  increased  by  the  fact  that  very  commonly 
they  are  much  distorted;  that  is,  some  faces  are  larger  than 
in  the  ideal  figure  and  others  smaller,  so  that  the  form  may 
be  like  that  of  Fig.  260  or  261;  other  still  more  irreg- 
ular crystals  are  not  uncommon.  In  all  these  cases,  how- 
ever, the  angles  remain  the  same  without  change,  notwith- 
standing the  seeming  irregularity  in  the  form.  In  trying 
to  put  such  a  crystal  into  its  proper  position,  it  may  be  re- 


276  MINERALS,  AND    HOW   TO   STUDY   THEM. 

membered  that  the  prismatic  faces  are  almost  always  finely 
lined,  or  striated,  in  a  horizontal  direction  (see  Fig.  3, 
p.  15). 

Though  crystals  like  those  shown  in  Figs.  249,  250,  hav- 
ing both  extremities  complete,  sometimes  occur,  it  is  much 
more  common  to  find  the  prisms  attached  at  one  end  and 
only  the  other  end  freely  developed,  as  shown  in  Figs.  251 
to  263;  further,  the  crystals  may  be  slender  and  tapering 
as  in  Fig.  251.  Not  infrequently  the  crystals  are  so  small 
over  a  surface  that  their  form  can  be  distinguished  only 
by  a  magnifying-glass,  and  the  surface  is  rough  and  drusy. 
There  are  also  many  massive  kinds  of  quartz,  as  mentioned 
in  subsequent  paragraphs. 

Twin  crystals  of  quartz  of  the  ordinary  type,  as  Fig.  125, 
p.  60,  are  not  often  found;  the  grouping  is  usually  quite 
irregular.  At  the  same  time  it  is  common  to  find  by  care- 
ful study  of  the  small  modifying  planes,  by  examination 
in  polarized  light  or  other  means,  that  a  crystal  of  quartz, 
appearing  at  first  sight  simple,  is  really  made  up  of  two 
crystals  in  twinning  position  interpenetrating  each  other 
very  irregularly.  Sometimes  this  is  indicated  by  the  dif- 
ference in  luster,  or  the  striations,  of  parts  of  the  surface 
of  the  prismatic  and  pyramidal  planes;  this  is  true,  for 
example,  with  many  of  the  crystals  of  smoky  quartz  from 
the  Pike's  Peak  region  in  Colorado. 

The  hardness  of  quartz  is  7,  so  that  it  cannot  be 
scratched  by  a  knife,  while  it  easily  scratches  glass;  the 
specific  gravity  of  pure  crystals  is  2.66.  The  luster  of 
quartz  crystals  is  vitreous  or  glassy;  some  massive  kinds 
are  greasy  and  others  waxy;  while  the  impure  kinds,  like 


DESCRIPTION   OF   MINERAL   SPECIES.  277 

jasper,  are  dull.  The  color  varies  widely;  crystals  are 
usually  colorless  or  nearly  so,  but  also  yellow  and  brown 
to  nearly  black;  there  are  also  pink,  green,  and  red  kinds. 
The  massive  forms  vary  still  more,  and  the  color  is  often 
in  bands  or  clouds,  as  described  under  the  varieties  below. 

Quartz  appears  under  a  great  number  of  varieties,  dif- 
fering particularly  in  color  and  structure,  and  as  many 
have  long  been  used  for  ornamental  purposes,  they  have 
received  a  number  of  distinct  names.  The  crystalline 
varieties  are  named  chiefly  according  to  their  color;  they 
include: 

Rock-crystal,  the  clear  colorless  variety;  when  quite  free 
from  flaws  it  is  used  for  spectacle-glasses  (as  the  Brazilian 
pebble).  The  Japanese  make  beautiful  crystal  spheres  of 
rock-crystal. 

Smoky  quartz,  having  a  smoky  brown  color,  sometimes 
very  dark.  It  is  cut  into  ornaments,  as  in  Switzerland, 
where  it  is  found  in  beautiful  specimens.  In  Scotland  it  is 
called  cairngorm  stone. 

Amethyst,  a  fine  purple  kind,  also  used  for  ornaments. 
A  yellow  variety  of  quartz  crystal  is  called  false  topaz. 

The  small  rock-crystals  are  so  bright  that  they  are  often 
called  diamonds,  though  really  far  inferior  to  the  diamond 
in  luster  and  brilliancy.  Lake  George  diamonds,  Bristol 
diamonds,  and  other  like  names  have  been  given  to  such 
crystals  of  quartz. 

Besides  the  distinct  crystals,  the  massive  quartz  is  some 
times  clear  as  crystal,  and  is  then  called,  like  the  crystals 
themselves,  rock-crystal.     It  is  also  sometimes  cloudy;  or 
again  milky  white,  then  called  milk-quartz;  or  rose-col- 


278  MINERALS,  AND   HOW  TO   STUDY   THEM. 

ored,in  rose-quartz.  Further,  aventurine  quartz  is  a  kind 
spangled  with  scales  of  hematite  (or  goethite)  or  mica. 
Cat's-eye  may  be  mentioned  here,  though  in  part  chalced- 
ony; it  is  a  kind  giving  when  polished  a  peculiar  effect 
of  opalescence,  usually  due  to  fibers  of  asbestus,  which 
somewhat  resembles  the  reflection  from  the  eye  of  a  cat. 
The  same  name  is  given  to  other  stones  having  like  ef- 
fects ;  the  highly-prized  cat's-eye  of  jewelry  is  a  variety  of 
chrysoberyl  (p.  242).  Tiger-eye  is  a  siliceous  stone,  usu- 


268. 


Chalcedony.  Agate. 

ally  of  a  yellow  color  and  somewhat  like  cat's-eye  in  effect; 
this  is  due  to  the  fibrous  structure  of  the  original  mineral 
<fcrocidolite)  from  which  it  has  been  formed  (see  p.  298). 

The  imperfectly  crystalline,  or  cryptocrystalline,  kinds  of 
quartz  are  also  numerous  and  include  many  valued  orna- 
mental stones.  . 

Chalcedony  is  a  kind  having  a  waxy  luster,  either  trans- 
parent or  translucent,  and  varying  in  color  from  white  to 
gray,  blue,  brown,  and  other  shades:  it  often  has  a  mam- 


DESCRIPTION   OF   MINERAL   SPECIES.  279 

millary,  botryoidal,  or  stalactitic  structure  (Fig.  262,  also 
Fig.  137,  p.  68). 

Agate  is  a  variegated  chalcedony,  commonly  with  the 
colors  arranged  in  delicate  parallel  bands,  straight,  curved, 
or  zigzag.  These  lines  often  follow  the  irregular  outline 
of  the  cavity  in  which  the  silica  was  deposited,  and  hence 
show  the  successive  layers  formed  (Fig.  263).  The  colors 
may  be  also  in  irregular  clouds,  as  in  clouded  agate.  Moss- 
agate  is  a  kind  of  chalcedony,  or  it  may  be  rock-crystal, 
containing  brown  or  black  mosslike  or  dendritic  forms 
264. 


Onyx. 

distributed  rather  thickly  through  the  mass;  these  consist 
of  some  metallic  oxide  (as  of  manganese),  and  have  noth- 
ing more  to  do  with  vegetation  than  the  frost  figures  on  a 
window-pane  in  winter.  Another  kind  of  agate  is  the 
ruin  or  fortification  agate. 

Carnelian  is  a  red  or  brownish-red  chalcedony;  sard  is 
essentially  the  same  stone. 

Onyx  is,  like  much  agate,  made  up  of  layers  of  different 
colors,  usually  white  and  black  or  white  and  brown,  but 
the  banding  is  straight  and  the  layers  are  in  even  planes. 


280  MINERALS,  AND   HOW   TO   STUDY   THEM. 

The  stones  may  hence  be  used  for  cameos,  the  head  being 
cut  from  one  layer  and  the  background  formed  by  the 
other.  Both  the  varieties  agate  and  onyx  are  often  arti- 
ficially colored  in  order  to  make  them  more  attractive  for 
ornaments. 

Sardonyx  is  like  onyx,  but  has  layers  of  sard  ^carnelian) 
with  others  which  are  white  or  black.  Prase  is  a  trans- 
lucent leek-green  chalcedony,  chrysoprase  an  apple-green 
chalcedony.  Heliotrope,  or  blood-stone,  is  green  (like  the 
kind  called  plasma)  with  spots  of  red  jasper  looking  a 
little  like  drops  of  blood. 

The  varieties  of  quartz  which  follow  are  all  more  or  less 
impure;  their  luster  is  dull,  and  they  are  in  many  cases 
nearly  or  quite  opaque. 

Jasper  is  an  impure  opaque  colored  quartz,  often  red 
(colored  by  red  iron  sesquioxide),  also  brown  or  ocher- 
yellow  and  dark  green.  Sometimes  the  red  and  green 
colors  are  shown  in  the  same  specimen,  arranged  in  bands 
as  in  riband  jasper. 

Flint  is  nearly  opaque  and  has  a  dull  color,  usually  gray, 
smoky  brown,  and  brownish  black.  The  exterior  is  often 
whitish,  from  mixture  with  lime  or  chalk,  in  which  it  is 
imbedded,  as  in  the  chalk  formation;  luster  barely  glisten- 
ing, subvitreous.  It  breaks  with  a  deeply  conchoidal  frac- 
ture and  a  sharp  cutting  edge,  and  is  hence  easily  chipped, 
as  by  the  Indians,  into  arrow-heads  or  hatchets.  Horn- 
stone  resembles  flint,  but  is  more  brittle,  and  the  fracture 
more  splintery.  Chert  is  a  term  often  applied  to  horn- 
gtone,  and  to  any  impure  flinty  rock. 

Basanite,  Lydian  Stone,  or  Touchstone  is  a  velvet-black 


DESCRIPTION   OF   MINERAL   SPECIES.  281 

flintlike  stone  used  on  account  of  its  hardness  and  black 
color  for  trying  the  purity  of  gold.  The  color  left  on 
the  stone  after  rubbing  the  metal  across  it  indicates  the 
amount  of  alloy.  It  is  not  splintery  like  hornstone. 

Silicified  wood  consists  largely  of  chalcedony,  agate,  or 
jasper,  which  has  replaced  the  woody  structure  of  the  tree; 
it  may  vary  much  in  color  and  thus  give  beautiful  effects 
when  polished,  as  is  true  of  the  much-used  kinds  from 
the  "  petrified  forest "  near  Holbrook,  Arizona.  Cavities 
often  contain  crystals  of  quartz.  Some  silicified  wood 
properly  belongs  to  the  species  opal. 

Quartz,  as  already  stated,  is  the  oxide  of  silicon,  or  sili- 
con dioxide,  SiOa.  It  is  infusible  before  the  blowpipe 
alone;  it  dissolves  slowly  in  a  borax  bead  and  with  effer- 
vescence in  a  bead  of  sodium  carbonate.  It  is  insoluble  in 
acids. 

Quartz  occurs  as  one  of  the  essential  constituents  of 
granite,  syenite,  gneiss,  mica  schist,  and  many  related 
rocks;  as  the  principal  constituent  of  quartz-rock  and 
many  sandstones;  as  an  unessential  ingredient  in  some 
trachyte,  porphyry,  etc.;  as  the  vein-stone  in  various  rocks, 
and  for  a  large  part  of  mineral  veins;  as  a  foreign  mineral 
in  the  cavities  of  trap,  basalt,  and  related  rocks,  some  lime- 
stones, etc.,  making  geodes  of  crystals,  or  of  chalcedony, 
agate,  carnelian,  etc.;  as  imbedded  nodules  or  masses  in 
various  limestones,  constituting  the  flint  of  the  chalk  for- 
mation, the  hornstone  of  other  limestones — these  nodules 
sometimes  becoming  continuous  layers;  as  masses  of  jasper 
occasionally  in  limestone.  It  is  the  principal  material  of 
the  pebbles  of  gravel-beds,  and  of  the  sands  of  the  sea- 


282  MINERALS,  AND   HOW  TO   STUDY   THEM. 

shore  and  sand-beds  everywhere.  The  famous  localities 
are  so  numerous  that  it  would  be  impossible  to  mention 
even  a  small  part  of  them.  The  finest  specimens  usually 
come  from  cavities  in  granite  and  related  rocks  as  in 
Switzerland.  North  Carolina  and  Colorado  afford  beauti- 
ful and  complex  crystals. 

Some  of  the  uses  of  the  various  varieties  of  quartz  have 
been  alluded  to;  most  of  them  are  in  the  way  of  orna- 
ments or  ornamental  stones.  Besides  this,  quartz-sand  is 
used  for  sandpaper  and  in  making  glass  and  porcelain; 
blocks  of  quartzite  are  employed  for  hearthstones. 

Opal 

OPAL  is  a  form  of  silica  containing  a  few  per  cent  of 
water.  It  does  not  occur  in  crystals,  but  in  massive  and 
amorphous  forms  only;  these  often  show  a  peculiar  effect 
called  opctlescence,  like  that  of  water  containing  a  few 
drops  of  milk,  and  some  varieties  show  a  beautiful  play  of 
color. 

The  hardness  is  about  6,  a  little  less  than  that  of  quartz, 
and  the  specific  gravity  is  also  less,  or  only  about  2.  The 
color  varies  from  white  to  yellow,  red,  brown,  green,  gray, 
and  black.  It  is  sometimes  transparent,  but  more  com- 
monly only  translucent.  It  is  soluble  in  caustic  alkalies, 
which  is  not  true  of  crystallized  quartz. 

The  most  beautiful  variety  of  opal  is  that  called  precious 
opal,  much  admired  because  of  the  delicate  play  of  colors, 
due  to  the  optical  effect  of  internal  reflections;  the  colors  are 
often  seen  on  a  white,  also  on  a  red,  ground.  One  kind  of 
precious  opal  with  a  bright  red  flash  of  light  is  called  the 


DESCRIP110H  OP  MINERAL  SPKCIES.  283 

fire-opal,  and  another  kind  is  the  harlequin-opal.  A  beau- 
tiful opal  found  in  Queensland  shows  an  iridescent  blue 
sometimes  like  the  effect  of  a  peacock's  wing. 

Common  opal  does  not  exhibit  this  play  of  colors,  and  it 
varies  widely  in  color  and  appearance.  Milk-opal  is  one 
kind,  which  has  a  pure  white  color  and  milky  opalescence; 
while  resin-opal  or  wax-opal  has  a  waxy  luster  and  yellow 
color.  Jasper-opal  is  intermediate  between  jasper  and 
opal.  Wood-opal  is  petrified  wood  in  which  the  mineral 
material  is  opal  instead  of  quartz.  Hyalite  or  Miiller's 
glass  is  a  kind  of  opal  in  clear  glassy  globular  forms  look- 
ing like  drops  of  gum. 

The  silica  deposited  from  hot  springs,  as  by  the  geysers 
of  the  Yellowstone  Park  and  those  of  New  Zealand,  is  a  kind 
of  opal  and  is  called  geyserite  or  siliceous  sinter  (Fig.  265). 

265. 


It  is  usually  white  or  gray  in  color,  translucent  or  opaque, 
and  often  with  a  pearly  luster  on  the  surface.  The  form 
is  varied  and  sometimes  very  beautiful.  Infusorial  earth 
is  a  kind  of  opal-silica  consisting  of  the  microscopic  shells 
of  the  minute  vegetable  organisms  called  diatoms,  and  as 
found  in  beds  sometimes  of  great  extent  is  properly  ranked 
as  a  mineral  and  a  variety  of  opal ;  electro-silicon  is  the 
trade  name  of  one  kind  much  used  for  polishing  silver:  the 


284  MINERALS,  AND   HOW  TO   STUDY   THEM. 

siliceous  shells  are  so  fine  that  they  do  not  scratch  the  sur- 
face. A  layer  of  pure  white  infusorial  earth  is  often  found 
under  a  peat-bed;  it  would  not  be  easy  to  estimate  the 
enormous  number  of  the  diatom  shells  that  would  be  re- 
quired to  make  up  a  cubic  inch  of  it. 

Opal  is  commonly  met  with  in  seams  of  certain  volcanic 
rocks;  it  sometimes  occurs  in  limestone  and  also  in  metal- 
lic veins.  The  Yellowstone  Park  is  a  famous  locality  for 
the  kind  called  geyserite. 

SILICATES. 

The  silicates,  as  has  already  been  remarked  (see  pp.  113, 
272),  form  a  very  large  class  of  minerals,  in  which  silicon  is 
present  as  the  acid  element,  and  various  metals,  sometimes 
iron,  manganese,  zinc,  copper,  but  more  often  aluminium, 
calcium,  magnesium,  sodium,  potassium,  are  the  bases.  As 
many  of  the  silicates  contain  two  or  more  of  the  bases  it  is 
more  convenient  to  treat  them  here  together  than  to  try 
to  separate  them  under  the  heads  of  the  metals  present; 
moreover,  very  few  of  them  are  economically  useful  as  ores 
of  the  metals  which  they  contain :  a  few  of  these  have  al- 
ready been  included  in  the  preceding  pages. 

THE  FELDSPARS. 

The  FELDSPARS  form  the  most  important  group  among 
the  silicates.  They  are  found  in  large  masses  in  what  are 
called  granite  veins— aggregations  of  feldspar  with  quartz 
and  mica  filling  veins  in  the  enclosing  rock.  They  are 
also  an  essential  part  of  nearly  all  the  common  kinds  of 
crystalline  rocks,  as  granite,  syenite,  gneiss,  also  basalt, 


DESCRIPTION   OF   MINERAL   SPECIES.  285 

trachyte,  and  others.  They  are  all  silicates  of  alumina 
with  either  potash,  soda,  lime,  or  rarely  baryta. 

The  feldspars  include  several  different  species.  Of 
these,  orthoclase,  or  potash  feldspar,  is  the  most  common; 
albite,  or  soda  feldspar,  is  also  common,  while  the  lime  feld- 
spar, anorthite,  is  rare,  more  so,  indeed,  than  the  inter- 
mediate kinds,  oligoclase  and  labradorite,  both  of  which 
are  soda-lime  feldspars.  There  is  also  a  kind  of  feldspar 
called  liyalopliane,  which  is  a  baryta  feldspar,  but  too  rare 
to  be  more  than  mentioned  here. 

All  the  feldspars  occur  in  crystals  which  have  a  certain 
general  resemblance  to  each  other,  although,  while  ortho- 
clase is  monoclinic,  albite,  anorthite,  oligoclose,  and  labra- 
dorite are  triclinic.  They  all  have  cleavage  in  two  direc- 
tions making  angles  of  90,°  or  nealy  90,°  with  each  other; 
hence  rectangular  cleavage  masses  are  very  common;  a 
little  attention,  however,  shows  that  the  two  directions  are 
unlike,  that  is,  cleavage  is  easier  parallel  to  one  face  than 
to  the  other.  They  have  a  hardness  about  6,  so  that  they 
are  not  scratched  by  a  knife.  The  specific  gravity  lies  be- 
tween 2.5  and  2.7,  or  is  not  far  from  that  of  quartz;  only 
for  hyalophane  is  the  specific  gravity  as  high  as  2.8.  The 
color  is  usually  not  far  from  white,  and  pale  yellowish, 
reddish,  or  greenish  shades  are  common,  while  dark  colors 
are  rare. 

Orthoclase.     Potash  Feldspar,  KAlSi,08. 

ORTHOCLASE,  or  potash  feldspar,  is  the  commonest  and 
most  valuable  species  of  the  group;  it  is  the  feldspar  of 
granite  and  granite  veins,  gneiss,  syenite,  and  many  other 


286 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


rocks.  The  name  is  from  two  Greek  words  (op$6s,  erect, 
and  /cAa'crz?,  fracture),  referring  to  the  existence  of  the 
two  prominent  cleavages  at  right  angles  to  each  other. 


These  cleavages  are  parallel  to  the  top  or  basal  plane  (c) 
and  to  the  side  plane  (b)  of  the  crystals,  as  shown  in  the 
figures  ('266  to  270).  These  figures  represent  the  common 
forms:  a  prism  of  nearly  120°(lettered  m),  the  faces  of  which 
271.  are  sometimes  quite  short  (Figs.  268, 

271);  a  basal  plane  (c)  inclined  116°  to  the 
front  edge  of  this  prism,and  another  plane 
or  two  planes  below  (and  behind)  lettered 
x  and  ?/;  the  angle  between  c  and  x  is 
129°  44',  and  between  c  and  y  is  99°  42'; 
also  x  is  inclined  114^°  to  the  front  edge 
of  m,  and  y  144J°  to  this  edge.  There  may  also  be  various 
other  modifying  planes,  as  a  pyramid  o  (Fig.  268),  a  prism 
z  (Figs.  267,  268),  a  dome  n  inclined  about  135°  to  c  (Fig, 
268). 

Twin  crystals  are  very  common,  especially  of  the  kind 
shown  in  Figs.  269,  270,  called  Carlsbad  twins,  because 
found  at  Carlsbad,  Bohemia;  notice  that  Fig.  269  looks 
like  a  simple  crystal,  and  it  is  only  by  carefully  observing 
the  fact  that  the  cleavage  exists  parallel  only  to  one  half  of 
the  end  face  above  and  below  (the  parts  lettered  c)  that  the 


DESCRIPTION   OF   MINERAL   SPECIES.  28? 

compound  character  is  proved.  Here  the  faces  c  and  x  of 
the  two  half-crystals  in  the  twinned  position  almost  coin- 
cide because  each  makes  nearly  the  same  angle  with 
the  front  edge  of  m.  Orthoclase  is  also  found  in  masses, 
sometimes  very  large,  usually  showing  very  distinctly  the 
two  cleavages  which  have  been  mentioned;  there  are  kinds, 
however,  which  are  close  and  compact  like  porcelain. 

The  hardness  of  orthoclase  is  6,  and  the  specific  gravity 
about  2.6;  the  hardness  is  the  commonest  test  by  which  it 
is  distinguished  from  calcite  and  barite,  while  it  is  also 
much  less  dense  than  barite.  The  luster  is  vitreous,  though 
sometimes  pearly  on  the  basal  cleavage  surface;  it  is  some- 
times clear  and  colorless,  but  commonly  it  is  white  or  red- 
dish, or  pale  yellow. 

The  clear,  glassy  variety  found  in  the  Alps  is  called 
adularia,  while  sanidine  is  a  glassy  kind  found  in  crystals 
imbedded  in  lava  and  various  volcanic  rocks;  moonstone 
is  a  kind  giving  a  beautiful  bluish  opalescence,  especially 
when  polished,  and  hence  used  for  pins  and  other  orna- 
ments; the  best  is  found  in  Ceylon;  some  moonstone  also 
belongs  to  the  species  albite.  The  formula  of  orthoclase  is 
stated  above,  KA1S308  or  K,O.Ala03.6SiOa,  which  gives 
the  percentage  composition:  Silica  (Si02)  64.7,  alumina 
(Al,03)  18.4,  potash  (K,0)  16.9  =  100.  It  fuses  on  thin 
edges  with  some  difficulty;  it  is  not  attacked  by  acids. 
On  the  method  required  to  show  the  presence  of  potash 
see  p.  134. 

Orthoclase,  as  has  been  mentioned,  is  the  common  feld- 
spar of  granite,  gneiss,  and  related  rocks;  it  is  present 
sometimes  in  large  masses  in  granite  veins,  as  in  the  New 


288  MINERALS,  AND   HOW  TO   STUDY   THEM. 

England  States,  also  North  Carolina  and  elsewhere.  In 
the  latter  case  it  is  often  possible  to  obtain  it  in  consider^ 
able  quantity  free  from  the  associated  quartz  and  mica, 
and  it  is  then  mined  and  used  in  the  making  of  porcelain. 
In  cavities  in  these  rocks  fine  crystals  can  frequently  be 
found.  Associated  with  the  orthoclase  feldspar  in  the 
granite  veins  is  commonly  the  soda  feldspar,  albite;  also 
often  many  interesting  minerals,  as  tourmaline  (sometimes 
the  rare  red  and  green  kinds),  beryl,  apatite,  the  rarer 
species  columbite,  cassiterite,  chrysoberyl,  spodumene,  and 
many  others. 

There  is  also  another  kind  of  potash  feldspar,  like  ortho- 
clase in  composition,  but  distinguished  by  optical  exami- 
nation under  the  microscope  as  a  triclinic  species;  the 
angle  between  the  two  cleavage  surfaces  differs  a  very  little 
from  90°.  This  kind  is  called  MICROCLINE,  and  includes 
the  beautiful  bluish-green  potash  feldspar  called  amazon- 
stone  from  near  Pike's  Peak  in  Colorado,  in  Siberia,  and 
elsewhere.  The  distinction  between  orthoclase  and  micro- 
cline,  however,  is  one  which  only  a  skilled  mineralogist 
can  note,  or,  perhaps,  is  bound  to  regard. 

Albite.     Soda  Feldspar,  NaAlSi308. 

ALBITE,  or  soda  feldspar,  takes  its  name  from  the  Latin 
albus,  meaning  white,  referring  to  the  fact  of  the  common 
color.  It  occurs  in  crystals  which  are  somewhat  like  ortho- 
clase in  angles,  although  belonging  to  the  triclinic  system 
(see  Fig.  99,  p.  46).  The  crystals,  however,  are  seldom 
distinct  and  easy  to  decipher.  Usually  they  are  small, 
flattened  in  one  direction  and  crowded  together  in  parallel 


DESCRIPTION    OF   MINERAL   SPECIES,  289 

lines  or  ridges,  a  little  resembling  barite  (p.  262).  Much 
albite  is  simply  massive,  and  then  the  cleavage  surface 
parallel  to  the  base  commonly  shows  a  series  of  fine  lines 
or  ridges  which  catch  the  light  when  the  specimen  is  held 
so  as  to  reflect  it  (Fig.  272).  These  are  due  to  a  kind  of 
twinning  (explained  on  p.  59),  in  which  the  parts  of  the 
crystal  are  in  thin  parallel  plates  alternately  repeated  in 
the  twinned  position.  This  kind  of  twinning  is  common 
in  all  the  triclinic  feldspars,  called  in  general  plagioclase,* 

272. 


in  which  the  two  cleavages  are  inclined  a  few  degrees  from 
the  90°  angle  characteristic  of  orthoclase.  The  massive 
albite  has  often  a  wavy  surface,  so  that  the  directions  of 
cleavage  are  more  or  less  concealed. 

The  hardness  of  albite  is  6  to  6.5,  and  the  specific  gravity 
about  2.62.  The  luster  is  vitreous  or  pearly,  and  the  color, 
besides  the  common  white,  also  bluish,  gray,  reddish,  green- 
ish; the  crystals  are  often  clear  and  glassy. 

The  formula  NaAlSi30.  or  NaaO.Al.,0,.6SiOa  gives  the 
percentage  composition:  Silica  (SiOJ  68.7,  alumina  (AlaO) 

*  From  the  Greek  nXdyioS,  oblique,  and  xvlacns,  fracture. 


290  MINERALS,  AND   HOW   TO   STUDY  THEM. 

19.05,  soda  (Na.,0)  11.8  =  100.  Albite  fuses  without  much 
difficulty  (more  easily  than  orthoclase)  and  colors  the  flame 
yellow.  It  is  not  attacked  by  acids. 

Albite  is  present  in  many  crystalline  rocks,  particularly 
in  granite  and  in  granite  veins,  where  it  occurs  with  the 
potash  feldspar,  orthoclase;  from  this  source  it  is  often 
obtained  in  fine  groups  of  crystals  in  cavities,  while  the 
massive  kinds  often  enclose  some  of  the  rare  species  men- 
tioned on  p.  288. 

Anorthite.     Lime  Feldspar,  CaAl2Si308. 

ANOETHITE,  or  lime  feldspar,  is  a  rare  species,  and  the 
best  crystals  occur  in  the  volcanic  rocks  of  Vesuvius.  It 
is  also  found  in  massive  forms,  but  they  can  be  identified 
only  by  minute  optical  examination  or  by  analysis. 

Anorthite  is  interesting  because  it  forms  one  end  of  a 
series,  at  the  other  end  of  which  is  albite,  or  soda  feldspar, 
and  which  includes  a  number  of  intermediate  species  con- 
taining both  soda  and  lime,  or  soda-lime  feldspars.  The 
full  understanding  of  this  subject  belongs  to  the  higher 
study  of  mineralogy,  but  it  will  be  easily  remembered  that 
between  the  pure  soda  feldspar,  albite,  with  68.7  per  cent 
of  silica  (SiO,),  and  the  pure  lime  feldspar,  anorthite,  with 
43.2  per  cent  Si05,  there  are  intermediate  kinds  varying  in 
the  amount  of  soda  and  lime.  Those  near  anorthite  have 
less  silica  and  less  soda,  and  those  near  albite  relatively 
more  of  both.  The  two  most  important  of  the  interme- 
diate kinds  are  oligoclase  and  labradorite.  OLIGOCLASE  is 
a  soda-lime  feldspar,  while  labradorite  is  rather  a  lime-soda 
feldspar.  Oligoclase  is  seldom  in  distinct  crystals,  and 


DESCRIPTION   OF   MINERAL  SPECIES.  291 

the  massive  kinds  can  be  identified  only  by  chemical  analy  • 
sis  or  6y  careful  optical  study. 

LABRADORITE  is  also  rare  in  crystals,  but  it  is  particu- 
larly interesting  because  it  often  shows  a  beautiful  play  of 
colors  in  certain  directions,  which  is  due  to  the  optical 
effect  of  a  multitude  of  minute  plates  of  foreign  species 
inclosed  in  the  specimen  and  which  are  seen  when  a  thin 
section  is  examined  under  the  microscope.  The  finest  lab- 
radorite  was  brought  to  Europe  from  Labrador  by  a  mis- 
sionary in  1770  and  was  then  called  Labrador  feldspar. 
The  best  specimens  still  come  from  the  same  source, 
but  many  of  the  rocks  of  the  Adirondack  region  consist 
largely  of  labradorite  and  pretty  specimens  may  be  oc- 
casionally found. 

The  following  species  are  allied  to  the  feldspars  in  the 
way  in  which  they  occur  in  certain  rocks: 

LEUCITE  is  a  silicate  of  alumina  and  potash  crystallizing 
usually  in  trapezohedrons,  like  the  common  form  of  garnet 
(Fig.  285,  p.  300).  The  color  is  white;  luster  vitreous; 
hardness  5.5  to  6;  specific  gravity  2.5. 

SODALITE  is  a  silicate  of  alumina  and  soda,  containing 
also  chlorine.  It  belongs  to  the  isometric  system,  but  is 
usually  found  in  massive  forms  varying  in  color  from  white 
to  gray,  yellow,  or  blue.  Hardness  5.5  to  6;  specific 
gravity  2.3. 

LAPIS-LAZULI  is  a  mineral  of  a  beautiful  blue  color,  and 
hence  often  used  as  an  ornamental  stone;  the  artificial 
ultramarine  is  similar.  It  is  a  silicate  of  alumina,  lime, 
and  soda,  containing  also  some  sulphur  trioxide  (SO,). 

NEPHELITE  is  a  silicate  of  alumina,  soda,  and  potash 


292 


MINERALS,  AND    HOW  TO   STUDY   THEM. 


occurring  in  glassy  hexagonal  crystals  and  in  massive 
forms  or  a  gray  to  green  or  red  color  and  with  greasy 
luster  (the  variety  elceolite).  Hardness  5.5  to  6;  specific 
gravity  2.6.  It  is  common  in  igneous  rocks. 

CANCRINITE  is  related  to  nephelite,  but  yields  also  car- 
bon dioxide  (COJ  and  water,  its  composition  being  rather 
complex.  Massive  forms  of  a  yellow  color  are  most  com- 
mon. Hardness  5  to  6;  specific  gravity  2.45. 

Pyroxene. 

PYROXENE  is  one  of  the  most  important  of  the  silicates, 
and  at  the  same  time  often  one  of  the  most  difficult  to 
recognize.  This  is  largely  because  it  appears  in  a  great 

273.  274.  275.  276.  277. 


variety  of  forms,  and  of  many  of  these  the  characters  may 
not  be  very  distinct. 

It  crystallizes  in  oblique  rhombic  prisms  (m),  having  an 
angle  in  front  (m  on  m  over  a)  of  87°  and  on  the  side  (over 
b)  of  93°.  If  a  and  b  are  small  or  absent,  a  crystal  looks 
at  first  sight  like  a  square  prism  and  may  be  mistaken  for 
this  if  the  end  planes  are  broken  off  or  indistinct.  On  the 
other  hand,  if  the  faces  of  the  prism,  m,  are  small  or  ab- 
sent, the  pinacoid  planes  a  and  b  being  prominent  (as  in 
Fig.  274,  and  Fig.  93  on  p.  45),  the  resemblance  to  a  square 


DESCRIPTION   OF   MINERAL   SPECIES.  293 

prism  is  even  more  striking,  since  a  and  b  actually  make 
angles  of  90°  with  each  other.  Here  also  the  monoclinic 
symmetry  in  the  distribution  of  the  terminal  planes  is 
decisive.  For  when  these  planes  (u,  v,  etc.)  are  distinct,  it 
is  easy  to  see  that  the  like  faces  are  present  only  two  and 
two,  not  four  at  each  end,  and  also  that  the  basal  plane  c 
is  oblique  to  the  face  a  and  to  the  prismatic  edge,  not  at 
right  angles  to  them.  Parallel  to  this  plane  c  the  crystal 
often  separates  into  thin  layers,  and  the  oblique  lines  show- 
ing this  structure  can  often  be  seen  on  the  vertical  faces. 

The  angle  of  the  prism  m  in  front  is  87°,  as  alreadj 
stated,  while  c  is  inclined  105°  50'  to  the  front  edge  of 
this  prism  or  to  the  face  a;  the  angle  between  two  faces  u 
is  131F;  of  o  95°;  and  of  s  121°.  See  Figs.  273  to  277, 
also  Figs.  93,  96,  p.  45. 

The  hardness  of  pyroxene  is  5.5  to  6,  and  the  specific 
gravity  varies  between  3.2  and  3.5.  The  luster  is  vitreous 
and  often  dull,  and  the  color  is  usually  a  dingy  gray  or 
green  to  black;  there  are  rare  kinds  which  are  transparent 
and  nearly  colorless,  also  white  varieties,  and  others  that 
are  bright  green  and  even  pink,  but  these  last  are  less 
common. 

The  nearly  colorless,  white,  or  very  pale  green  kind  of 
pyroxene — usually  called  diopside — is  a  silicate  of  lime 
and  magnesia  and  has  the  formula  CaMgSi.,0,  or 
CaO.MgO.2SiO.,.  There  is  also  a  rather  rare  black  kind  that 
contains  lime  and  iron,  and  another,  more  common,  con- 
taining lime,  magnesia,  and  iron,  which  has  usually  a  gray- 
i«h  green  or  dark  green  color.  Salite  belongs  here;  it  often 
shows  a  lamellar  structure  parallel  to  the  basal  plane  (c). 


294  MINERALS,  AND   HOW  TO   STUDY   THEM. 

Diallage  is  a  thin  foliated  variety  of  pyroxene  of  a  green 
color.  Uoccolite  is  about  the  same  as  salite  in  composition, 
but  occurs  in  grains  easily  separated  from  each  other; 
they  are  often  imbedded  in  crystalline  limestone.  It  is 
aamed  from  the  Greek  KOKKOS,  a  grain. 

There  are  also  some  important  kinds  of  pyroxene  which 
contain  alumina,  but  not  in  very  large  amount,  and  the 
commonest  of  these  is  called  augite.  This  is  greenish 
black  or  brownish  black  in  color,  or  even  quite  black;  the 
specific  gravity  is  about  3.3;  it  contains  silica,  lime,  mag- 
nesia, and  iron,  as  well  as  some  alumina.  This  is  the  com- 
mon pyroxene  of  eruptive  rocks,  like  basalt  and  diabase,  to 
which  belong  the  trap  rocks  of  the  Palisades  on  the  Hud- 
son and  many  localities  in  Connecticut,  Massachusetts,  and 
elsewhere. 

All  the  varieties  of  pyroxene,  except  those  last  men- 
tioned containing  alumina,  have  the  general  formula  of  a 
metasilicate,  RSi03,  in  which  R  represents  calcium,  mag- 
nesium, also  iron  and  manganese.  Most  kinds  of  pyroxene 
fuse  in  the  blowpipe  flame  without  great  trouble;  the  dark- 
colored  kinds  give  a  magnetic  bead,  from  the  presence  of 
iron. 

Pyroxene  is  a  common  mineral  in  crystalline  limestone 
and  dolomite,  especially  the  white  and  light-colored  kinds; 
also  in  serpentine  and  in  various  types  of  volcanic  rocks. 
It  is  curious  to  note  that  the  name  (from  nvp,  fire,  and 
Zeros,  stranger)  early  given  by  a  French  mineralogist 
recorded  his  idea  that  it  was  "a  stranger  in  the  domain  of 
fire,"  while  in  fact  exactly  the  reverse  is  true.  Some  of 
the  best  localities  for  crystals  of  pyroxene  are  in  the 


DESCRIPTION   OF   MINERAL  SPECIES.  206 

nortnern  part  of  the  State  of  .New  \ork;  also  at  many 
points  in  Canada. 

ENSTATITE  and  HYPERSTHENE  are  minerals  related  to 
pyroxene  in  form  and  composition,  but  crystallizing  in  the 
orthorhombic  system.  Enstatite  is  a  silicate  of  magnesia 
with  but  little  iron,  while  hypersthene  contains  iron  in  a 
larger  amount.  Hypersthene  is  especially  interesting  be- 
cause some  varieties  show  a  peculiar  almost  metallic  luster 
in  certain  directions,  due  to  internal  reflections  from  en- 
closed microscopic  scales  of  a  foreign  mineral.  Bronzite 
is  a  variety  of  eustatite  containing  more  iron  than  the  other 
kinds;  it  is  fusible  with  difficulty  in  very  thin  splinters, 
while  the  magnesian  enstatite  is  nearly  infusible.  The 
alteration  of  enstatite  at  Edwards,  New  York,  has  yielded 
the  fibrous  talc  alluded  to  on  a  subsequent  page. 

SPODUMENE  is  a  rather  rare  mineral  also  related  to 
pyroxene,  interesting  as  being  a  silicate  of  alumina  and 
lithia  (LiAlSia08  or  LiilO.AlaOs.4SiOa).  It  occurs  usually 
in  white,  grayish,  or  yellowish  prismatic  crystals,  some- 
times of  enormous  size  (even  up  to  four  feet  or  more  in 
length),  as  at  Chesterfield,  Mass.,  and  Branchville,  Conn. 
It  is  also  rarely  a  fine  purple,  and  the  most  beautiful 
variety  (called  liiddenite],  occurring  in  small  crystals  oi 
a  fine  emerald-green,  has  been  used  as  a  gem;  this  kind  is 
found  in  North  Carolina.  Spodumene  fuses  before  the 
Mowpipe  at  3.5  and  gives  a  fine  purple-red  color  to  tha 
flame. 

WOLLASTONITE,  a  silicate  of  lime,  is  also  related  to  pyrox- 
ene with  the  formula  CaSiOs.  It  is  usually  found  in 
white  cleavable  fragments,  showing  a  fibrous  structure,  also 


896 


MINERALS.  AND   HOW  TO   STUDY  THEM. 


in  flat  monoclinic  crystals,  whence  it  is  sometimes  called 
tabular  spar. 

Amphibole. 

AMPHIBOLE  is  a  mineral  which  in  many  respects  is 
closely  related  to  pyroxene,  both  as  regards  form  and  com- 
position; it  resembles  it  further  in  the  common  green  and 
gray  shades  of  color.  It  also  occurs  in  monoclinic  crystals, 
but  these  show  perfect  cleavage  parallel  to  the  faces  of  the 
prisrn  (m),  which  make  an  angle  in  front  of  124^°.  The 
crystals  are  very  often  terminated  by  two  planes  oblique 
to  the  prismatic  edge  and  making  a  large  angle  with  one 
another  (rr  =  148£°,  Figs.  278-280).  The  form  of  the 


278. 


279. 


crystals  and  the  large  angle  of  prismatic  cleavage  serve 
to  distinguish  them  from  crystals  of  pyroxene. 

Besides  the  crystals,  it  is  common  to  find  amphibole  in 
columnar  and  bladed  forms,  which,  however,  still  show  the 
prismatic  cleavage  to  the  skilled  observer,  and,  as  forms 
like  these  are  rare  with  pyroxene,  this  serves  to  aid  in  dis- 
tinguishing the  two  species.  Further,  it  may  be  coarse  to 
fine  fibrous,  and  in  the  latter  case  look  like  flax. 

The  hardness  is  5  to  6,  and  the  specific  gravity  varies 
from  2.9  to  3.4.  The  luster  is  vitreous,  or  pearly  in  the 
fibrous  kinds.  The  color  varies  from  white  to  gray,  green 


DESCRIPTION   OF   MINERAL   SPECIES.  297 

of  many  shades,  and  black,  also  occasionally  pink.  Some 
kinds  phosphoresce,  or  give  out  light,  as  when  two  pieces 
are  rubbed  together  in  the  dark. 

The  common  varieties  correspond  pretty  closely  to  those 
of  pyroxene.  Tremolite  is  a  silicate  of  lime  and  magnesia 
free  from  iron,  and  is  white  in  color,  or  very  pale  green, 
like  the  related  diopside.  Actinolite  is  green,  and  besides 
lime  and  magnesia  contains  also  iron;  it  is  often  in  fibrous 
or  columnar  masses,  sometimes  with  a  radiated  structure. 
Nephrite,  including  much  of  the  green  Oriental  jade,  is  a 
closely  compact  variety  of  actinolite;  the  whitish  varieties 
of  nephrite,  however,  approach  nearer  to  tremolite.  An- 
other kind  of  jade  is  a  different  mineral,  though  having 
the  same  appearance;  it  is  a  silicate  of  alumina  and  soda, 
related  to  pyroxene,  and  is  called  jadeite. 

Asbestus*  is  a  variety  usually  like  actinolite  in  compo- 
sition, but  separable  into  very  fine  flexible  fibers,  which 
may  be  made  or  woven  into  an  incombustible  paper  or 
cloth.  Mountain  leather  is  a  curious  kind,  in  thin  flexible 
but  very  tough  sheets,  consisting  of  interlaced  fibers;  it  is 
usually  nearly  white  in  color. 

There  is  also,  as  with  pyroxene,  a  variety  which  contains 
alumina,  and  is  dark  green  or  brown  to  black  in  color; 
this  variety  is  called  common  hornblende  (a  name  some- 
times given  to  the  whole  species)  and  corresponds  to 
augite  under  pyroxene.  Manganese  is  also  present  in  cer- 

*  A  fibrous  variety  of  serpentine  is  also  called  asbestus;  it  is 
easily  distinguished,  because  it  coutains  about  fourteen  per  cent  of 
water;  it  is  described  on  a  later  page,  and  includes  most  of  the 
asbestus  milled  for  use  in  the  arts. 


298  MINERALS,  AND   HOW  TO   STUDY   THEM. 

tain  kinds  of  amphibole.  Before  the  blowpipe  amphibole 
behaves  essentially  like  pyroxene. 

Amphibole  occurs  in  crystalline  limestone  (the  variety 
tremolite);  also  associated  with  talc  or  serpentine  (actino- 
lite) ;  in  granitic  rocks  and  in  certain  volcanic  rocks  (horn- 
blende). It  also  forms  rock-masses,  as  hornblende  rock, 
hornblende  schist,  etc.  At  many  localities  it  occurs  asso- 
ciated with  pyroxene. 

CKOCIDOLITE,  also  called  blue  asbesttis,  is  related  to  am- 
phibole. It  occurs  in  fine  fibrous  forms  of  a  delicate  blue 
color.  The  fibers  often  penetrate  quartz  and  then  make  a 
handsome  ornamental  stone,  especially  when  by  alteration 
they  have  become  yellow  (being  changed  to  limonite). 
This  is  the  well-known  tiger-eye  brought  in  large  quan- 
tities from  South  Africa. 

Beryl.     Beryllium  silicate,  Be3AlaSi60lg. 

BERYL  is  one  of  those  species  which  are  almost  always 
in  distinct  crystals  and  usually  in  forms  easy  to  recognize; 
it  is  also  interesting  because  some  varieties  are  used  as 
a  gem.  The  crystals  are  hexagonal  prisms,  sometimes 
quite  slender,  again  stout,  and  occasionally  very  large;  it  is 
only  rarely  that  they  show  any  terminal  planes  (Figs.  281, 
282,  see  also  Figs.  69-71,  p.  38). 

The  hardness  of  beryl  is  7.5  or  a  little  above  that  of 
quartz,  and  on  this  account  and  because  of  the  beautiful 
color  it  sometimes  has  it  ranks  as  one  of  the  precious 
stones;  the  specific  gravity  is  2.7.  The  luster  is  vitreous 
or  glassy, — in  this  respect  also  it  resembles  quartz, — and 
the  color  is  usually  some  shade  of  green:  bluish  green  in 


DESCRIPTION   OF   MINERAL   SPECIES. 


299 


common  beryl,  clear  mountain-green  in  the  variety  called 
aquamarine,  and  a  deep  emerald-green  in  the  highly  prized 
variety,  emerald.  There  are  also  light  or  dark  yellow  kinds 
sometimes  having  a  rich  golden  color,  and  occasionally 
white  and,  still  more  rare,  pink  kinds. 

The  formula  BesAlaSi,018  or  3BeO.Al,08.6SiOa  gives  the 
percentage  composition  :  Silica  (SiOJ  67.0,  alumina 
(A1,0S)  19.0,  glucina  (BeO)  14.0  =  100.  The  color  of  the 

281. 


emerald  is  usually  attributed  to  a  minute  amount  of  chro- 
mium. It  is  infusible  before  the  blowpipe  and  is  not 
attacked  by  acids. 

Beryl  is  found  in  granite  rocks,  with  feldspar  and 
quartz;  the  granite  veins  of  New  England  and  North 
Carolina  often  afford  it  in  beautiful  specimens.  The 
beryls  of  Acworth,  N.  H.,  are  sometimes  as  large  as  a  barrel. 
The  finest  emeralds  come  from  near  Muso  in  Colombia, 
South  America;  others  are  found  in  the  Ural,  and  some  in 
North  Carolina. 

The  element  BERYLLIUM — also  called  Glucinum — which 
is  prominent  in  the  species  beryl,  is  a  rare  one  and  is  only 
known  in  a  small  number  of  other  species,  none  of  them 
common.  Of  these  chnjsolenjl  (BeO.AlaO, ,  p.  242)  and 
the  silicate,  euclase,  are  the  most  important;  both  of  these 


300 


MINERALS,  AND   HOW   TO    STUDY   THEM. 


are  used  as  gems;  there  are  also  several  other  silicates,  of 
which  pheuacite  (Be2Si04)  occurring  in  glassy  rhombo- 
hedral  crystals  is  the  most  important;  also  two  rare  phos- 
phates of  beryllium  (herder  He  and  beryllonite). 

Garnet. 

GARNET  is  another  species  which,  like  beryl,  is  almost 
always  in  distinct  crystals,  and  as  these  crystals  are  com- 
monly isolated  and  scattered 
through  the  rock,  it  is  not 
difficult  to  recognize  them. 
There  are,  however,  massive 
kinds  needing  some  skill  for 
their  identification;  these  are 
occasionally  used  in  the  same 
way  as  emery,  though  much 
less  hard. 

The  crystals  are  usually  twelve-sided,  having  the  form 
of  a  rhombic  dodecahedron  (Fig.  284),  or  twenty-four- 
sided  and  then  called  a  trapezohedron  (Fig.  285).  There  are 
also  combinations  of  these  two  forms  (Fig.  286),  as  also  of 


Garnet  crystals  in  mica  schist. 


284. 


285. 


287. 


the  dodecahedron  with  the  planes  of  a  hexoctahedron  (Fig. 
287).  It  should  be  remembered  that  the  dodecahedron 
has  angles  of  120°  between  two  adjacent  faces,  while  these 


DESCRIPTION   OF   MINERAL   SPECIES.  301 

planes  themselves  are  diamond  in  shape  with  plane  angles 
of  60°  and  120°.  The  trapezohedron  (n)  has  quadrilateral 
faces,  and  the  angles  over  the  two  kinds  of  edges  are  131° 
49'  and  146°  27'. 

The  hardness  of  garnet  is  7  to  7.5,  and  the  specific 
gravity  varies  from  3.2  to  4.3.  The  luster  is  vitreous,  and 
the  color,  while  most  commonly  red,  varies  also  from  the 
colorless  kinds  to  those  which  are  yellow,  brown,  black, 
and  green. 

The  composition  of  the  different  kinds  of  garnet  varies 
widely — as  shown  in  the  following  list — and  with  this  the 
color  and  specific  gravity  also  change;  the  form,  however, 
is  the  same  for  all  kinds. 

The  chief  kinds,  or  subspecies  as  they  are  in  fact,  with 
their  formulas  are  given  here;  but  between  these  there  are 
also  many  intermediate  varieties: 

Grossularite,  Lime-alumina  Garnet,  3CaO.Al303.3SiOa. 
Pyrope,  Magnesia-alumina  Garnet,  SMgO.AljOg.SSiO,,. 
Almandite,  Iron-alumina  Garnet,  3FeO.AlaOs.3SiO.,. 
Spessartite,  Manganese-alumina  Garnet,3M)iO.Al.1Oa.3SiO.J. 
Andradite,  Lime-iron  Garnet,  3CaO.Fe,Oa.3Si09. 

Uvarovite,      Lime-chromium  Garnet,     3CaO.Cra08.3SiOa. 

The  kind  called  grossulnrite  is  a  silicate  of  lime  and  alu- 
mina; it  may  be  colorless,  white,  pale  yellow,  or  green,  also 
brownish  yellow  or  cinnamon-brown,  and  occasionally  rose- 
red.  The  specific  gravity  is  3.55  to  3.66.  The  commonest 
kind  is  brownish  red,  and  is  called  cinnamon-stone  or  hes- 
sonite.  The  original  grossular  garnet  is  green;  it  was 
named  from  the  botanical  name  for  the  gooseberry. 


302  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Pyrope,  or  magnesia  garnet,  is  a  silicate  of  magnesia  and 
alumina;  it  often  has  a  deep  red  color,  and  when  perfectly 
clear  is  used  as  a  gem  and  called  precious  garnet.  The 
specific  gravity  is  3.7  to  3.75. 

Almandite,  or  almandine  garnet,  includes  a  large  part  of 
the  common  garnet,  the  rest  belonging  to  andradite.  It  is 
a  silicate  of  iron  and  alumina  and  is  ordinarily  red  in  color, 
but  sometimes  black.  When  clear  so  that  it  can  be  cut 
into  gems  it  is,  like  pyrope,  also  called  precious  garnet. 
The  specific  gravity  varies  from  3.9  to  4.^,  increasing  with 
the  amount  of  iron  present. 

Spessartite,  or  manganese  garnet,  is  a  rarer  kind.  It  is 
a  silicate  of  manganese  and  alumina  and  has  a  brownish- 
red  or  hyacinth-red  color.  The  specific  gravity  is  4.0  to 
4.3.  One  kind  found  in  Virginia  is  used  as  a  gem,  it  being 
perfectly  transparent  and  of  a  peculiar  shade  of  red. 

Andradite,  or  the  common  iron  garnet,  is  a  silicate  of 
lime  and  iron.  It  varies  in  color  from  pale  yellow  to 
apple-green  or  emerald-green,  also  to  red,  brown,  and 
black.  Much  of  the  common  garnet  belongs  here,  and 
from  this  fact  it  is  obvious  that  alrnandite  and  andradite 
can  be  surely  separated  and  identified  only  by  analysis. 
Specific  gravity  3.8  to  3.9.  Topazolite  is  a  topaz-yellow 
variety;  melanite  is  black;  demantoid  from  Siberia,  which 
is  a  beautiful  green,  is  used  as  a  gem. 

Uvarovite,  or  chrome  garnet,  is  a  rare  kind  of  a  fine 
emerald-green  color  and  distinguished  in  composition  by 
its  containing  lime  and  chromium.  Specific  gravity  3.5. 

The  common  kinds  of  garnet  fuse  easily  and  give  a 
light  brown  or  black  glass;  the  latter  is  magnetic  if  much 


DESCRIPTION   OF  MINERAL   SPECIES.  303 

iron  is  present.  The  borax  beads  give  reactions  for  iron 
;n  most  cases,  and  often  also  for  manganese. 

Garnet  commonly  occurs  in  crystals  scattered  through 
granite,  gneiss,  or  mica  schist  (almandite  or  andradite), 
•a';so  in  crystalline  limestone  (grossularite) ;  with  serpentine 
(pyrope)  or  chromite  (uvarovite);  also  in  some  volcanic 
rocks  (melanite,  rarely  spessartite). 

Related  to  garnet  are  the  following  rare  species: 

HELVITE,  a  silicate  of  beryllium,  manganese,  and  iron, 
also  yielding  sulphur;  it  crystallizes  in  tetrahedral  forms, 
color  commonly  yellow. 

DANALITE,  in  octahedral  crystals  or  masses  of  a  flesh- 
red  to  gray  color;  composition  near  that  of  helvite,  but 
contains  also  zinc. 

EULYTITE,  a  silicate  of  bismuth,  occurring  in  yellow  to 
gray  tetrahedral  crystals. 

ZUNYITE,  a  tetrahedral  fluo-silicate  of  aluminium. 

THE   MICAS. 

The  MICAS  are  characterized  before  all  by  their  very 
perfect  cleavage,  in  consequence  of  which  they  admit  of 
being  split  into  leaves  much  thinner  than  a  sheet  of  paper 
— in  fact  it  is  difficult  to  set  any  limit  to  the  extent  to 
whioh  this  process  may  be  carried.  These  leaves  or  sheets 
are  usually  very  elastic  and  spring  back  with  force  when 
bent,  but  there  are  kinds  of  mica  in  which  this  elasticity 
'.&  wanting  and  sometimes  the  leaves  are  brittle.  The 
natural  plates  of  mica  have  usually  the  form  of  either  a 
rhomb  with  angles  of  120°  and  60°,  or  a  hexagon,  all  the 
angles  being  120°.  The  micas  are  silicates  of  alurairv 


304  MINERALS,  AND    HOW  TO   STUDY   THEM. 

with  potash,  rarely  soda  or  lithia,  also  magnesia,  iron,  and 
some  other  elements. 

The  most  important  species  is  muscovite,  or  potash 
mica;  somewhat  less  so  is  biotite,  the  magnesia-iron  mica, 
to  which  phlogopite  is  closely  related,  and  lepidolite  or 
lithia  mica  is  a  rather  rare  though  interesting  species. 

Muscovite.     Potash  mica,  (H,K)AlSi04. 

MUSCOVITE  is  the  common  mica  which  in  the  form  of 
clear  or  slightly  smoky-colored  plates  is  used  for  the  open- 
ings of  stoves  and  lanterns  and  even  for  the  windows  of 
houses  in  some  regions  where  glass  is  difficult  to  obtain; 
it  was  this  last  use  in  Russia  that  gave  the  name  to  the 


mineral  of  Muscovy  glass,  whence  the  mineralogical  name 
of  Muscovite. 

Muscovite  commonly  occurs  in  scales  or  sheets  without 
any  regular  form,  its  crystallization  having  been  con- 
strained by  the  surrounding  quartz  or  feldspar.  In  favor- 
able cases,  however,  distinct  crystals  are  found  which  on 


DESCRIPTION   OF   MINERAL   SPECIES.  305 

top  may  have  the  form  of  either  a  rhomb  with  angles  of 
60°  and  120°,  or  one  with  the  acute  angles  cut  off  (Fig. 
288),  or  it  may  be  a  regular  hexagon.  The  sides  of  the 
crystals  usually  taper  sharply  and  are  rough  with  the 
edges  of  the  plates;  this  makes  them  look  dark,  but  more 
light  can  pass  through  in  this  direction  than  in  the  other 
perpendicular  to  the  perfect  basal  cleavage.  The'  struct- 
ure of  the  sheets,  even  when  they  have  no  regular  shape, 
conforms  to  this  hexagonal  outline;  this  is  shown  by  the 
289. 


fact  that  when  a  point  not  too  sharp  is  held  against  a  sheet 
and  a  blow  struck  with  a  hammer,  a  six-rayed  star  with 
branches  intersecting  at  angles  of  60°  is  the  result  (see  the 
center  of  Fig.  289).  These  branches  are  parallel  to  the 
sides  of  a  crystal  having  a  hexagonal  outline.  The  same 
thing  is  shown  by  the  fact  that  the  transparent  magnetite 
(Fig.  289)  which  is  sometimes  found  in  muscovite  forms  a 
network  along  lines  having  the  same  direction. 

Besides  the  kinds  which  are  in  distinct  crystals  or  in 


306  MINERALS,  AND   HOW   TO   STUDY   THEM. 

plates  there  are  others  made  up  of  minute  scales,  some- 
times aggregated  into  compact  forms  or  those  with  a 
featherlike  structure  as  in  plumose  mica.  The  plates  or 
scales  are  sometimes  arranged  in  spherical  forms  like 
balls,  and  these  occasionally  have  also  a  radiated  structure. 

The  cleavage  has  already  been  spoken  of  as  perfect  par- 
allel to  the  top  or  basal  plane.  The  hardness  of  mica  on 
the  cleavage  surface  is  only  2  to  2.5,  but  the  edges  are 
somewhat  harder;  the  specific  gravity  is  2.76  to  3.  The 
luster  is  vitreous,  but  often  silvery  and  pearly.  The  color 
depends  upon  the  thickness  of  the  sheet:  it  is  dark  brown 
when  rather  thick,  and  becomes  lighter  and  finally  color- 
less when  in  very  thin  sheets.  Smoky  brown  is  the  com- 
mon color.  There  are,  however,  yellow,  pink,  and  green 
kinds,  while  the  silvery  kind  was  long  ago  called  cat's  sil- 
ver and  the  yellower  scales  cat's  gold.  It  is  possible  for  a 
person  not  very  experienced  to  mistake  the  minute  yellow 
scales  scattered  through  a  rock  for  particles  of  gold,  but  a 
touch  with  a  knife  shows  the  difference. 

Muscovite  is  a  silicate  of  alumina  and  potash  having 
essentially  the  formula  KAlSi04  or  K,O.Al,0,.2Si09.  A 
little  water  is  usually  yielded  when  a  fragment  is  heated 
very  hot,  and  this  water  is  believed  to  be  formed  from  the 
hydrogen  in  the  compound  which  takes  the  place  of  part 
of  the  potassium.  Some  varieties,  which  give  off  water  more 
easily,  have  a  greasy  feel,  while  the  folia  have  little  elasticity. 
It  is  difficult  to  fuse,  and  usually  melts  only  on  very  thin 
edges.  It  is  not  decomposed  by  acids. 

Muscovite  is  commonly  found  in  scales  or  plates  in  gran- 
ite, gneiss,  and  similar  rocks.  In  what  are  called  granite 


DESCRIPTION   OF   MINERAL   SPECIES.  307 

veins,  where  the  ordinary  constituents  of  granite,  the  feld- 
spar, quartz,  and  mica  are  crystallized  out  in  large  masses, 
the  mica  is  sometimes  found  in  immense  sheets  a  yard  or 
more  across,  and  may  then  be  mined  with  success.  Tha^e 
granite  veins,  as  stated  on  p.  287,  are  very  interesting  to 
the  mineralogist,  for  in  them  he  looks  not  only  for  well- 
crystallized  specimens  of  the  three  essential  minerals 
named,  but  for  many  other  rarer  minerals,  as  often  tour- 
maline, beryl,  apatite,  garnet,  also  columbite,  samarskite, 
microlite,  uranium  compounds  and  others. 

Some  of  the  localities  of  these  granite  veins,  or  pegmatite 
veins  as  they  are  sometimes  called,  are  at  Paris  and  Hebron, 
Maine;  Acworth,  N.  H.;  Goshen  and  Chesterfield,  Mass.; 
Had  dam  and  Branchville,  Conn.  They  also  occur  in 
Pennsylvania  and  the  states  south,  especially  western 
North  Carolina,  where  the  mica  mines  are  famous  mineral 
localities;  further  in  Colorado,  S.  Dakota,  Canada. 

The  use  of  mica  for  stove-windows  and  lanterns  has 
been  spoken  of;  this  is  an  important  industry,  and  the 
mica  mines  of  North  Carolina  before  alluded  to  produce 
considerable  amounts.  Mica  is  also  brought  from  Green- 
land and  from  India.  Besides  this  most  important  use  of 
mica,  the  scales  too  small  to  oe  cut,  and  trimmings  of 
larger  sheets,  are  ground  to  powder  and  employed  to  give 
a  silvery  sheen  to  wallpaper. 

PINITE  is  a  mineral  related  to  common  potash-mica  in 
composition,  but  compact  in  structure  and  derived  from 
the  decomposition  of  some  one  of  a  number  of  other  min- 
erals, as,  for  example,  iolite,  nephelite,  scapolite,  etc. 


308  MINERALS,  AND   HOW  TO  STUDY  THEM. 

Biotite.     Magnesia-iron  Mica. 

BIOTITE  is  the  second  most  important  kind  of  mica.  It 
includes  most  of  the  dark  green,  brown,  or  black  mica 
found  in  granite  and  other  rocks,  especially  those  of  vol- 
canic or  igneous  origin.  It  occurs  usually  in  crystals  with 
hexagonal  outline  and  in  irregular  imbedded  scales. 

The  hardness  is  about  2.5  to  3,  like  that  of  muscovite, 
but  the  specific  gravity  is  sometimes  a  little  higher,  2.7  to 
3.1.  The  luster  is  bright  and  pearly  on  the  cleavage  sur- 
face, but  vitreous  on  the  edges.  The  color,  as  already 
stated,  ranges  between  green  or  brown  to  black.  There  is 
a  wide  variation  in  color,  however,  corresponding  to  the 
variation  in  composition. 

Some  biotite — besides  silica  and  alumina — contains  mag- 
nesia and  potash  with  but  little  iron,  while  other  kinds 
contain  much  iron  and  almost  no  magnesia;  the  iron  kinds 
have  the  darker  color.  A  mica  close  to  biotite  and  con- 
taining a  large  amount  of  iron  is  called  lepidomelane,  be- 
cause found  in  black  scales. 

PHLOQOPITE  is  a  mica  near  biotite  and  sometimes  re- 
garded as  only  a  variety  of  it.  It  occurs  in  six- 
sided  crystals,  often  rough  tapering  prisms 
(Fig.  290),  and  is  found  usually  in  limestone  or 
in  serpentine  rocks,  and  often  has  a  copper-red 
color  on  the  cleavage  surface.  To  phlogopite 
belongs  the  interesting  star-mica  of  northern 
New  York  and  Canada,  a  brown  mica  which  shows 
a  fine  six-rayed  star  when  a  candle  flame  is  viewed 
through  it.  This  phenomenon  is  called  asterism,  and 


DESCRIPTION    OF   MINERAL   SPECIES.  309 

is  explained  by  the  presence  of  minute  rodlike  crystals 
(often  of  rutile)  enclosed  in  the  mica  and  lying  in  positions 
parallel  to  the  six-rayed  star  obtained  by  a  blow.  Some- 
times a  twelve-rayed  star  is  seen. 

Lepidolite.     Lithia  Mica. 

LEPIDOLITE,  or  lithia  mica,  is  another  kind  of  mica,  but 
not  a  common  one.  It  is  often  found  in  masses  seeming 
to  have  a  close  granular  structure,  but  really  made  up  of 
minute  scales,  and  this  has  given  it  the  name  from  the 
Greek  (henis,  a  scale).  The  scales,  however,  are  some- 
times large,  and  occasionally  lepidolite  is  found  in  plates 
like  other  micas. 

The  color  is  commonly  lilac  or  pink,  and  this  kind  of 
mica,  the  granular  kind  especially,  is  often  found  with 
prismatic  crystals  of  lithia  tourmaline  imbedded  in  it, 
making  specimens  of  great  beauty.  In  consequence  of 
the  presence  of  lithia  this  mica  yields  a  red  flame  when 
heated  with  the  blowpipe.  Specimens  of  this  kind  come 
from  Maine,  also  from  California;  there  is  a  foreign  locality 
in  Moravia. 

The  VERMICULITES  include  a  series  of  mica-like  min- 
erals, which  separate  by  cleavage  into  soft  pliable  and  in- 
elastic leaves.  The  luster  is  pearly  or  bronzelike,  and  the 
color  usually  yellow  or  brown.  They  contain  a  consider- 
able amount  of  water,  and,  on  this  account,  when  heated  so 
that  the  water  is  driven  off,  the  leaves  open  out,  or  exfoliate, 
into  peculiar  wormlike  threads  of  the  most  curious  appear- 
ance. The  force  of  expansion  is  often  sufficient  to  break  a 
glass  tube  in  which  the  fragment  is  being  heated.  This 


310  MINERALS,  AND   HOW  TO   STUDY    THEM. 

property  has  given  the  name  from  the  Latin,  vermiculan, 
to  breed  worms.  The  vermiculites  have  been  derived  from 
the  alteration  of  some  of  the  micas  (as  biotite  or  phlogo- 
pite),  or  the  chlorites,  so  that  the  composition  is  often 
rather  indefinite,  and  hence  the  mineralogist  does  not 
think  very  highly  of  them. 

To  the  micas  are  related  several  rather  rare  species,  of 
which  the  most  important  is  MARGARITE,  or  pearl  mica; 
others  are  chloritoid,  ottrelite,  etc.  They  are  sometimes 
called  brittle  micas,  because  the  folia  are  rather  brittle. 
Margarite  has  a  pearly  luster  on  the  cleavage  surface  and 
a  grayish  or  pinkish  color;  it  is  a  silicate  of  alumina  and 
lime,  and  is  often  associated  with  corundum.  Ottrclite 
usually  occurs  in  small  brown  scales  in  a  rock  called 
"  ottrelite-schist". 

THE   CHLORITES. 

The  CHLORITES  are  like  the  micas  in  many  respects: 
thus  in  basal  cleavage,  in  the  occurrence  in  scales  or  plates, 
and  in  the  hexagonal  outline  of  these  plates.  They  are 
easily  distinguished,  however,  for  the  plates  even  when 
large  show  little  or  no  elasticity,  but  when  bent  finally 
break.  The  name  chlorite  is  from  the  Greek  word  for 
green  (^Aopo?),  the  same  word  which  has  given  the  name 
to  the  yellow-green  gas  chlorine,  but  there  is  no  other 
relation  between  the  chlorites  and  chlorine.  Besides  the 
green  chlorites  there  are  others  that  are  rose-red. 

The  chlorites  are  silicates  of  alumina,  iron,  and  magnesia 
with  water,  the  amounts  of  the  different  elements  varying 
widely,  and  hence  a  number  of  species  are  included.  The 


DESCRIPTION"   OF   MINERAL   SPECIES.  311 

most  prominent  one  is  clinochlore,  also  sometimes  called 
ripidolite,  and  the  related  penninite.  Besides  these  crys- 
tallized chlorites  there  are  also  kinds  in  minute  scales  usu- 
ally bright  green  in  color,  and  others  that  are  quite  dense 
and  compact.  These  kinds  of  chlorite  are  found  in  rocks 
and  form  the  mass  of  chlorite  rock  and  chlorite  slate. 

Clinochlore. 

CLINOCHORE  is  the  most  distinctly  characterized  of  the 
chlorites  occurring  in  plates,  sometimes  large,  having  a 
fine  green  color  and  showing  the  characteristic  basal 
cleavage.  The  plates  have  a  hexagonal  outline  and  the 
crystals  sometimes  appear  as  roughly  hexagonal;  they  are 
truly  monoclinic,  however,  and  hence  291. 

the  name.  Figure  291  is  from  a 
drawing  of  a  large  crystal  of  clino- 
chlore (one  third  natural  size)  found 
nt  West  Chester,  Pennsylvania. 

The  hardness  of  clinochlore  is  2  to 
2.5,  and  the  specific  gravity  2.65  to 
2. 78.  Besides  the  bright  green  kinds 
there  are  others  which  are  rose-red  and  white.  It  yields 
Water  in  the  closed  tube  and  fuses  with  difficulty  in  the  for- 
ceps; a  reaction  for  iron  with  borax  is  ordinarily  obtained. 
Clinochlore  occurs  most  commonly  in  chloritic  and  talcose 
rocks  or  in  serpentine. 

PENNINITE  is  another  kind  of  chlorite  very  like  clino- 
chlore, in  fact  not  to  be  distinguished  from  it  in  compo- 
sition, but  only  in  the  form  of  the  crystals. 


312  MINERALS,  AND   HOW  TO  STUDY  THEM. 

Chrysolite,  or  Olivine.    (Mg,Fe),Si04. 

CHRYSOLITE  is  a  silicate  of  magnesia  and  iron  having  a 
vitreous  luster  and  bright  yellowish-green  color.  It  is 
hard,  H.  =  6.5  to  7,  and  also  of  high  specific  gravity,  Gr.  =  3.3 
to  3.6,  the  higher  values  belonging  to  the  kinds  with  most 
iron.  The  luster  is  vitreous,  and  the  color,  besides  that 
mentioned,  may  be  yellow  to  olive-green  and  brown.  The 
composition  is  (Mg,Fej,Si04  or  2(Mg,Fe)0,SiO,.  As  this 
method  of  writing  the  formula  indicates,  the  relative 
amounts  of  magnesia  and  iron  vary  widely,  from  kinds 
with  little  iron  (G-.  =  3.26)  to  those  containing  30  p.  c. 
(G.  =  3.57). 

Chrysolite  is  not  a  common  mineral,  but  it  is  an  impor- 
tant one  in  connection  with  a  number  of  eruptive  rocks, 
especially  basalt,  in  which  it  is  often  scattered  in  grains; 
it  is  also  a  common  mineral  in  some  meteorites.  One 
celebrated  specimen  found  in  Siberia  in  1749  consists  of  a 
spongy  mass  of  metallic  iron  containing  bright  yellow 
grains  of  chrysolite.  The  clear  yellow-green  kinds  of 
chrysolite,  as  those  from  the  Orient,  are  sometimes  used  as 
a  gem;  one  kind  found  in  Arizona  has  been  called  Job's 
tears  because  of  their  peculiar  pitted  appearance. 

Chrysolite  (golden-stone,  from  xpucros,  gold,  and  A/$o&, 
stone)  is  an  old  term  in  the  mineralogy  of  former  times, 
given  to  a  number  of  yellow  minerals  used  as  gems,  thus 
to  beryl,  topaz,  chrysoberyl,  zircon,  etc. 

Zircon.     Zirconium  silicate,  ZrSi04* 
ZIRCON  is  a  rather  rare  mineral  found  in  square  prisms 


DESCRIPTION-   OF   MINERAL   SPECIES. 


313 


or  pyramids,  commonly  quite  small.  In  these  the  angle 
between  the  prism  (m)  and  pyramid  (p)  is  132°  10',  and 
the  angle  between  two  adjacent  pyramidal  faces  (p]  is 
123°  20'  (Figs.  292  to  294);  the  crystals  are  sometimes  low 


292. 


293. 


pyramids,  and  again  long  prisms  terminated  by  the  pyram- 
idal planes.  Crystals  which  are  more  highly  modified  also 
occasionally  occur;  some  of  them  are  figured  on  p.  34, 
Figs.  56-58. 

The  hardness  of  zircon  is  7.5,  and  the  specific  gravity  is 
4.7.  The  luster  is  brilliant  adamantine,  and  the  color 
varies  from  colorless  through  varies  shades  of  reddish 
brown  and  yellow.  Its  high  luster  makes  it  when  clear 
prized  as  a  precious  stone;  i:i  fact  the  gem  called  hyacinth 
is  a  variety  of  zircon  having  a  reddish  color.  There  is  no 
very  distinct  cleavage,  but  the  fracture  is  conchoidal. 

Zircon  consists  of  silica  and  zirconia,  its  formula  being 
ZrSi04  or  ZrOa.SiO  ,  which  requires:  Silica  32.8,  zirconia 
67.2  =  100.  It  is  quite  infusible  and  not  attacked  by  acids. 

Zircon  is  sometimes  found  in  crystalline  limestone,  also 
in  various  crystalline  rocks.  In  some  regions  it  occurs  in 
the  rock  so  abundantly  that  when  the  rock  has  been  worn 
down  by  the  weather  it  is  left  unaltered  in  considerable 
quantities.  It  may  then  be  obtained  by  washing  the 
gravel  after  the  manner  of  the  gold-miner.  Fine  crystals 


314  MINERALS,  AND   HOW  TO    STUDY   THEM. 

are  obtained  in  New  York,  and  very  large  ones  in  Canada, 
as  in  Eenfrew  County;  good  specimens  also  come  from 
North  Carolina  and  Colorado  ;  Norway  and  the  Ural 
Mountains  are  famous  foreign  localities. 

THORITE  is  related  to  zircon,  having  the  same  crystal- 
line form,  while  in  composition  it  is  essentially  a  silicate  of 
thoria  (ThSiOJ ;  it  is,  however,  much  rarer.  Usually  it 
is  found  in  altered  and  hence  hydrated  forms.  Orangite 
is  an  orange-yellow  variety.  The  use  now  made  of  the 
earth  thoria  is  mentioned  under  the  description  of  mona- 
zite  (p.  271). 

Scapolite. 

SCAPOLITE  is  the  name  strictly  of  a  group  of  minerals, 
all  crystallizing  in  square  prisms,  and  consisting  of  silica, 
alumina,  lime,  and  soda,  but  varying  rather  widely  in  com- 
position, somewhat  as  do  the  feldspars. 

The  common  kind  of  scapolite  is  called  WERNERITE, 
alter  an  early  German  mineralogist.  It  is  found  in  crys- 
tals  having  the  form  of  a  square  prism,  or 
an  eight-sided  prism  formed  of  two  square 
prisms  (a  and  m  in  Fig.  295),  parallel  to 
each  of  which  there  is  good  cleavage.  The 
pyramid  r  has  an  angle  of  136°  15',  or  on 
m  of  121°  50'.  Massive  varieties  are  also 
common:  they  often  show  a  faint  fibrous  structure  OB 
the  surface.  The  hardness  is  5  to  6;  the  specific  gravitj 
2.7;  the  luster  is  vitreous  or  pearly,  and  the  color  com- 
monly varies  from  white  to  gray;  there  are  also  yellowish, 
reddish,  greenish  and  bluish  varieties. 


DESCRIPTION   OF   MINERAL  SPECIES.  315 

The  composition  of  a  typical  specimen  is  as  follows: 
Silica  (SiOJ  51.9,  alumina  (Al.O.)  26.5,  lime  (CaO)  12.9, 
soda  (NaaO)  7.2,  chlorine  (01)  2.0  =  100.5.  Scapolite 
fuses  easily  before  the  blowpipe  with  intumescence  to  a 
white  glass  full  of  bubbles  (hence  called  blebby).  It  is 
partially  decomposed  by  hydrochloric  acid.  The  form 
and  cleavage  are  the  most  distinctive  characters. 

MEIONITE,  DIPYRE,  and  MARIALITE  are  others  of  the 
scapolites. 

Vesuvianite,  or  Idocrase. 

VESUVIANITE  takes  its  name  from  Vesuvius,  because  the 
igneous  rocks  of  that  famous  volcano  have  long  furnished 
some  of  the  finest  specimens.  It  occurs  in 
tetragonal  crystals  of  varied  habit,  sometimes 
highly  complex;  a  simple  form  is  shown  in 
Fig.  296.  Here  the  angle  of  the  pyramid  p  is 
129°  20',  while  that  of  c  on  p  is  142°  45'. 
There  are  also  massive  forms,  either  columnar 
or  granular. 

The  hardness  is  6.5,  and  the  specific  gravity  3.4.  The 
luster  is  vitreous,  inclining  to  resinous,  and  the  color,  com- 
monly brown  to  green,  may  be  also  sulphur-yellow  and 
bright  blue;  transparent  crystals  occur,  but  are  not  the 
rule. 

The  composition  is  complex,  it  being  essentially  a  sili- 
cate of  alumina,  iron,  and  lime.  Before  the  blowpipe  it 
fuses  with  intumescence  at  3.  It  is  only  partly  decom- 
posed by  hydrochloric  acid.  It  is  found  most  commonly 
in  granular  limestone;  also  in  serpentine,  chorite  schist 


316 


MINERALS,  AND   HOW  TO    STUDY   THEM. 


297. 


and  related  rocks.  Commonly  associated  minerals  are 
grossularite  (garnet),  diopside  (pyroxene),  also  epidote, 
titanite,  etc.  It  resembles  some  brown  garnet  and  tourma- 
line (also  epidote),  but  is  more  fusible,  and  when  crystal- 
lized has  a  different  form. 

Epidote. 

EPIDOTE  is  found  usually  in  prismatic  crystals,  often 
very  slender  and  terminated  at  one  end  only;  they  belong 
to  the  monoclinic  system.  Fig. 
297  shows  a  common  type,  in 
which  the  angle  ca  =  115°  23', 
ar  =  128°  19',  nn  (at  the  end) 
=  109°  31'.  The  same  form 
is  represented  in  Fig.  298  in 
an  erect  position;  the  former. 

however,  is  that  which  shows  the  monoclinic  character. 
It  also  often  occurs  in  fibrous  or  columnar  kinds  and  others 
that  are  compact  and  granular.  The  crystals  show  perfect 
cleavage  parallel  to  the  face  c  and  imperfect  cleavage 
parallel  to  a. 

The  hardness  is  6  to  7,  and  the  specific  gravity  3.3  to 
3.5.  The  luster  is  vitreous,  and  the  color  commonly  green, 
although  there  are  black  and  pink  varieties;  the  peculiar 
yellow-green  or  pistache-green  of  ordinary  epidote  is  so 
characteristic  that  in  the  majority  of  cases  it  suffices  to 
distinguish  it  from  some  kinds  of  amphibole  and  from 
tourmaline,  both  of  which  minerals  it  resembles. 

Epidote  is  a  silicate  of  alumina,  iron,  and  lime,  but  it 
varies  rather  widely  in  composition,  especially  as  regards 


DESCRIPTION    OF   MINERAL   SPECIES.  317 

the  relative  amounts  of  alumina  and  iron.  It  fuses  with 
intumescence  rather  easily,  because  of  the  iron  it  contains, 
and  gives  a  magnetic  globule.  It  is  partially  decomposed 
by  hydrochloric  acid.  It  is  a  common  mineral  in  many 
crystalline  rocks,  as  gneiss,  mica-schist,  amphibole-schist, 
etc.  Beautiful  specimens  come  from  the  Alps;  in  the 
United  States  from  Haddam,  Conn.,  and  other  localities. 

Closely  related  to  epidote  is  the  rare  ALLANITE,  a  black 
mineral  containing  cerium  and  other  rare  elements;  also 
ZOISITE,  a  white  to  ash  gray,  greenish,  or  pale  red  mineral 
often  in  columnar  or  compact  masses.  It  is  like  epidote  in 
composition,  except  that  it  contains  almost  no  iron.  Be- 
fore the  blowpipe  it  swells  up  and  fuses  to  a  blebby  glass; 
after  ignition  it  gelatinizes  with  hydrochloric  acid. 

Tourmaline. 

TOURMALINE  is  one  of  the  most  attractive  minerals 
among  the  silicates;  its  varieties  show  a  greater  range  of 
color  than  any  other  species,  not  even  excepting  fluorite, 

299.  300.  301. 


and  some  of  the  clear  pink  and  green  kinds  make  beauti- 
ful gems. 

It  is  almost  always  found  in  prismatic  crystals,  bounded 
often  by  three  sides,  sometimes  by  six,  also  by  nine,  and 
not  infrequently  rounded  so  that  there  are  no  distinct 


318  MINERALS,  AND  HOW  TO  STUDY  THEM. 

faces  to  be  distinguished  at  all.  These  prisms  give,  when 
broken  across,  as  they  are  often  observed  in  the  enclosing 
quartz  or  other  rock,  outlines  which  are  usually  three-sided 
(Fig.  301),  also  six-sided  or  nine-sided.  It  belongs  to  the 
rhombohedral  system,  and  the  terminal  faces  are  conse- 
quently arranged  in  three  or  multiples  of  three.  The  com- 
monest form  is  the  obtuse  rhombohedron  (Figs.  299,  302), 
which  gives  a  terminal  angle  of  about  133°.  There  are 
also  many  more  complex  forms,  and  not  infrequently  when 
both  ends  of  a  crystal  are  finished  by  planes  (Fig.  300),  we 


find  them  unlike.  Corresponding  to  this  dissimilar  mo- 
lecular structure,  in  the  opposite  directions  of  the  vertical 
axis,  is  the  development  of  -j-  and  —  electricity  at  these 
ends  on  change  of  temperature  (see  pyro-electricity,  p.  97). 
The  prismatic  faces  are  usually  lined  or  striated  in  a  ver- 
tical direction,  and  sometimes  to  such  an  extent  that  the 
whole  crystal  is  rounded  (Fig.  302).  Radiating  groups  of 
crystals  (Fig.  303)  are  common.  Massive  kinds  commonly 
show  a  columnar  structure,  the  mass  appearing  as  if  made 
up  of  a  bundle  of  parallel  crystals;  there  are  also  varieties 
which  are  simply  compact.  No  distinct  cleavage  is  noted, 


DESCRIPTION   OF   MINERAL  SPECIES.  319 

but  the  fracture  varies  from  uneven  to  imperfectly  con- 
choidal.  It  is  brittle,  and  the  effect  of  the  fractured  black 
mass  is  often  like  a  piece  of  coal. 

The  hardness  of  tourmaline  is  7  to  7.5,  and  the  specific 
gravity  varies  from  3  to  3.2.  The  luster  is  vitreous,  and 
the  color,  as  remarked  above,  very  varied.  Black  is  the 
commonest  kind,  and  this  the  form  of  tourmaline  that  is 
found  so  often  in  granite.  There  are  also  kinds  of  tour- 
maline that  are  yellow  and  brown;  others  that  are  blue, 
green,  pink,  and  gray;  while  occasionally  the  crystals  are 
white  or  colorless.  It  is  not  uncommon  to  find  crystals  in 
which  the  color  differs  in  different  parts:  they  may  be 
pink  at  one  end  and  green  at  the  other,  or  there  may  be 
a  pink  center  surrounded  by  a  green  border,  and  so  on. 

Tourmaline  is  a  silicate  of  alumina  containing  iron, 
magnesia,  and  the  rare  element  boron,  also  lithia,  soda,  and 
potash  in  some  kinds.  There  are  a  number  of  distinct 
varieties  differing  in  color  and  in  composition.  Common 
black  tourmaline  is  an  iron  kind ;  the  brown  tourmaline  is 
a  magnesian,  while  the  beautiful  pink  variety  contains 
lithia.  This  last  kind,  called  rulellite,  is  often  associated 
with  the  lithia  mica,  lepidolite,  as  has  been  remarked 
before. 

The  brown  magnesian  varieties  fuse  rather  easily  to  a 
blebby  enamel,  while  the  iron  kinds  fuse  with  difficulty, 
and  the  lithia  variety  is  infusible.  If  first  powdered  and 
tnen  carefully  mixed  with  three  times  its  volume  of  potas- 
sium bisulphate  and  its  own  volume  of  fluorite  (all  in  pow- 
aer),  and  if  then  the  mixture  is  supported  on  a  platinum 
wire  in  the  blowpipe  flame,  it  colors  it  momentarily  green 


320  MINERALS,  AND   HOW  TO   STUDY  THEM. 

because  of  the  boron  liberated.  This  is  a  delicate  and 
important  test  (for  tourmaline  is  not  infrequently  a  diffi- 
cult mineral  to  recognize),  and  the  student  should  practice 
it  on  a  known  specimen  until  he  is  always  successful. 

Tourmaline  is  especially  common  in  granite  and  similar 
rocks,  but  it  also  occurs  in  crystalline  limestone;  it  is 
found  in  fine  specimens  at  many  localities  in  New  Eng- 
land, New  York,  New  Jersey,  Pennsylvania,  also  the  states 
to  the  south;  noted  foreign  localities  too  are  very  numer- 
ous, as  Elba,  Brazil,  etc.  The  black  needles,  prisms  or 
clusters  of  prisms,  sometimes  in  radiating  groups,  are  very 
characteristic;  they  are  hard  but  show  no  cleavage  and  have 
a  peculiar  fracture  like  that  of  coal.  It  is  distinguished 
by  these  characters  from  similar  varieties  of  amphibole 
and  epidote,  which  it  often  resembles.  The  boron  test  above 
described  is  conclusive  when  the  identification  is  otherwise 
doubtful. 

Topaz.     Aluminium  fluo-silicate. 

TOPAZ  is  another  gem  silicate,  beautiful  in  its  fine  crys- 
tals and  in  its  brilliancy  of  luster  and  color.  It  occurs  in 
prismatic  crystals,  terminated  by  rhombic  pyramids,  some- 
times acute,  sometimes  obtuse.  The  angle  of  the  common 
prism  (m)  is  124°  17',  but  another  prism  (I)  of  nearly  90° 
(86°  49')  is  also  common;  the  angle  uu  =  141°  0'  and 
oo  =  130°  23'  (both  in  front).  There  are  also  .coarse 
crystals  and  massive  or  fibrous  forms,  but  these  last  are 
not  so  common.  The  perfect  basal  cleavage  of  topaz  is 
one  of  its  most  characteristic  points.  Hard  as  it.  is,  it  ia 
easily  broken  in  a  direction  across  the  prism,  and  will  yield 
thin  plates  with  very  smooth  faces. 


DESCRIPTION    OF   MINERAL   SPECIES. 


321 


The  hardness  of  topaz  is  8,  and  the  specific  gravity  about 
3,56.  The  luster  is  vitreous,  and  the  color  varies  from 
colorless  to  white,  wine-yellow,  and  blue.  Pink  topaz  is 
scnietimes  found,  but  most  of  the  pink  is  obtained  by 
heating  the  deep  wine-yellow  crystals  from  Brazil. 

Topaz  is  a  silicate  of  alumina  containing  some  17  per 
cent  of  the  rare  element  fluorine;  the  formula  is  AlaSiOt, 
Ike  andalusite,  with,  however,  part  of  the  oxygen  replaced 
by  fluorine.  It  is  infusible  before  the  blowpipe,  but  gives 
a  fine  blue  when  ignited  after  being  moistened  with  cobalt 
304.  305. 


solution;  it  also  gives  the  reaction  for  fluorine  (p.  153) 
when  fused  in  the  closed  tube  with  salt  of  phosphorus. 

Topaz  is  commonly  found  in  gneiss  or  granite  with 
tourmaline,  mica,  beryl;  also  cassiterite  or  tin-stone,  apa- 
tite, fluorite.  Siberia  and  Brazil  yield  beautiful  speci- 
mens, also  the  tin-mining  regions  of  Saxony,  Bohemia, 
and  Cornwall.  A  notable  locality  in  this  country,  espe- 
cially for  very  large  crystals,  is  at  Stoneham,  Maine; 
another  at  Trumbull,  Conn.  It  sometimes  occurs  in  fine 
clear  crystals  in  volcanic  rocks,  as  in  Colorado,  Utah,  and 
Mexico. 

DANBURITE  is  a  silicate  of  calcium  and  boron  with  the 
formula  CaBa(Si04)a  or  CaO.BjO^SiO,  occurring  in  pale 


322  MINERALS,  AND   HOW  TO   STUDY   THEM. 

yellow  prismatic  crystals,  closely  resembling  topaz  in  habit 
and  angles;  also  in  imbedded  crystalline  grains.  The 
hardness  is  7  to  7.5;  specific  gravity  3.0;  luster  vitreous  to 
greasy.  It  fuses  in  the  forceps  (at  3.5)  and  gives  a  green 
flame  (boron). 

IOLITE,  or  Cordierite,  is  a  silicate  of  ammina,  iron, 
and  magnesia.  It  is  sometimes  found  in  prismatic  crystals, 
also  in  massive  forms  commonly  showing  a  fine  blue  color. 
Hardness  7  to  7.5;  specific  gravity  2.6.  Much  iolite  is 
altered  to  pinite  (p.  307). 

Titanite,  or  Sphene.     CaTiSiOB. 

TITANITE  is  a  silicate  of  calcium  containing  titanium. 
It  is  almost  always  in  crystals,  which  vary  much  in  form, 
though  often  showing  a  form  with  an  acute  edge,  which 
has  given  to  it  one  of  its  common  names, 
sphene  (from  the  Greek  crffrrjv,  ivedge). 
The  crystals  are  monoclinic  in  form  and 
of  very  varied  habit;  a  common  kind  is 
shown  in  Fig.  307;  angles  mm  =  113"  31', 
nn  =  136°  11',  mn  =  152°  46',  en  =  141° 
44'.     Massive  lamellar  or  compact  kinds  are  less  common 
than  the  distinct  crystals. 

The  hardness  is  5  to  5. 5,  and  the  specific  gravity  3.54. 
The  luster  is  resinous  to  adamantine,  and  the  common 
color  varies  from  yellow  to  brown  and  black;  there  are  also 
green  and  pink  kinds. 

The  formula  is  CaTiSi06  or  CaO,TiOa,SiO.,,  and  the  per- 
centage composition :  Silica  (SiOJ  30.6,  titanium  dioxide 
(TiO,)  40.8,  lime  (CaO)  28.6  =  100.  Iron  is  present  in  the 


DESCRIPTION   OF   MINERAL   SPECIES.  323 

darker-colored  kinds,  and  sometimes  also  manganese.  It 
fuses  with  intumescence  to  a  yellow,  brown,  or  black  glass. 
With  salt  of  phosphorus  it  gives  a  violet  bead  in  the  re- 
ducing flame  (titanium),  which  is  distinct  unless  the  min- 
eral contains  a  good  deal  of  iron. 

Titanite  is  not  a  common  mineral,  but  it  is  found  in  fine 
crystals,  as  in  limestone  with  apatite,  in  northern  New 
York;  also  in  very  large  crystals  in  Renfrew  County,  On- 
tario, Canada.  The  Alps  yield  beautiful  crystals. 

Andalusite.     Aluminium  silicate,  Al2SiOB. 

ANDALUSITE  is  a  silicate  of  alumina,  named  from  the 
first  locality  where  it  was  identified,  Andalusia  in  Spain. 
It  is  found  in  rhombic  prisms  having  an  angle  of  about 
90°.  The  crystals  are  usually  imbedded  and  seldom  dis- 
tinctly formed;  sometimes  they  are  hardly  distinct  at  all 
until  the  weather  has  removed  the  surrounding  rock  and 
left  the  harder,  more  resisting  crystals  protruding  from 
the  surface  as  ribs  or  veins.  Coarse  prismatic  to  columnar 
and  massive  forms  are  common. 

An  interesting  variety  of  andalusite  is  that  called  cliias- 
tolite  or  made.  In  this  there  are  parts  of  the  crystal  that 
are  white  and  others  that  contain  carbonaceous  impurities, 
and  are  hence  black;  often  these  are  regularly  arranged 
through  the  length  of  the  crystal,  giving  a  variety  of  forms 
on  the  cross-section,  as  shown  in  Fig.  114,  p.  55.  As  seen 
there  the  form  on  the  cross-section  is  a  little  like  the  Greek 
letter  x>  and  this  resemblance  has  given  it  the  name  chias- 
tolite. 

The  hardness  is  7.5,  and  the  specific  gravity  3.2.     The 


324  MINERALS,  AND   HOW  TO   STUDY  THEM. 

luster  is  vitreous,  and  the  color  varies  from  white  or  gray 
to  pink,  brown,  or  green. 

The  chemical  formula  is  Al2Si06  or  Al203.SiO.,,  and  the 
percentage  composition  is:  Silica  36.8,  alumina  63.2  =  100. 
It  is  infusible  before  the  blowpipe,  and  a  fragment  becomes 
blue  when  moistened  with  cobalt  solution  and  ignited. 
Andalusite  is  not  uncommon  in  crystalline  or  partly  crys- 
talline schists,  as  those  of  New  England ;  sillimanite  is  a 
frequent  associate.  In  the  White  Mountain  region  in  New 
Hampshire,  as  on  Mount  "Washington,  andalusite  is  con- 
spicuous in  rough  crystalline  forms. 

Cyanite  and  Sillimanite  are  two  other  minerals  having 
the  same  composition  as  andalusite. 

CYANITE,  named  from  the  Greek  for  blue  (xvavos), 
because  of  its  characteristic  color,  is  usually  found  in  thin- 
bladed  crystals,  showing  a  fine  blue  sometimes  over  the 
whole,  sometimes  as  a  central  strip  between  paler  or  even 
colorless  sides.  There  are  also  gray  and  green  varieties 
and  those  which  are  columnar  to  fibrous.  The  hardness  is 
only  5  on  the  flat  side  of  the  blades,  but  on  the  edges  a 
little  over  7;  specific  gravity  3.6;  luster  vitreous  to  pearly. 
Fine  specimens  come  from  North  Carolina. 

SILLIMANITE  occurs  rarely  in  prismatic  crystals;  usually 
it  is  in  fibrous  or  columnar  forms,  and  then  the  crystals 
are  not  distinct.  There  is  a  very  perfect  cleavage  in  a 
direction  parallel  to  the  length  of  the  prisms.  The  hard- 
ness is  6  to  7,  and  the  specific  gravity  3.2;  luster  vitreous 
or  subadamantine;  color  pale  brown  to  gray  and  green. 
It  is  found  in  crystalline  schists,  often  with  staurolite. 

PYROPHYLLITE  is  a  silicate  of  aluminium   containing 


DESCRIPTION   OF   MINERAL  SPECIES.  325 

water  (HaO.Al2Os.4SiOa).  It  often  occurs  in  foliated  masses 
with  radiated  lamellar  structure;  also  granular  to  compact 
(pencil-stone).  Hardness  1  to  2;  specific  gravity  2.8  to  2.9; 
luster  pearly;  color  white  to  green  or  yellow.  In  the  for- 
ceps all  but  the  compact  varieties  exfoliate,  but  fuse  only 
on  the  edges  with  difficulty  and  give  a  blue  (alumina)  with 
cobalt  solution. 

AXINITE  is  a  rather  rare  silicate  of  aluminium,  calcium, 
iron  and  manganese,  containing  also  boron.  It  occurs  in 
triclinic  crystals,  often  with  a  sharp  edge  (hence  named 
from  a^ivrj,  an  ax) ;  one  form  is  figured  on  page  46,  Fig. 
98.  Hardness  6.5  to  7;  specific  gravity  3.27;  luster  vitre- 
ous; color  clove-brown  to  yellow  and  gray,  also  blue.  It 
fuses  easily  and  gives  a  green  flame  in  the  forceps,  due  to 
the  boron. 

Staurolite. 

STAUROLITE,  or  Cross-stone,*  is  remarkable  for  the  vari- 
ety of  its  compound  or  twin  crystals.  The  simple  form 
is  a  rhombic  prism  with  the  angles  of  129°  20'  and  50°  40', 
and  with  these  a  dome,  d,  often  occurs,  making  an  angle 
of  124°  44'  with  the  top  plane.  This  is  not  rare,  but  it  is 
more  common  to  find  two  crystals  crossing  each  other, 
sometimes  at  right  angles  (Fig.  308,  also  Fig.  119,  p.  58), 
sometimes  at  an  angle  of  nearly  60°,  as  shown  in  Fig.  309; 
more  complex  twins,  of  the  same  interpeuetration  type, 
also  occur  where  three  or  even  four  crystals  are  grouped 
together. 

The  hardness  is  7  to  7.5,  and  the  specific  gravity  about 

*  The  name  is  derived  from  the  Greek  words  oravpoS,  a  cross, 
aud  Ar&oS,  a  stone. 


326  MINERALS,  AND    HOW   TO   STUDY   THEM. 

3.7;  the  fracture  is  subconchoidal,  and  cleavage  parallel  to 
the  side  plane  is  sometimes  noted.  The  luster  is  vitreous, 
and  the  color  reddish,  yellowish  brown,  brownish  black,  or 
gray. 

Staurolite  is  a  silicate  of  aluminium,  iron,  and  magnesium, 
but  its  formula  is  quite  complex.  It  occurs  especially 
with  garnet,  tourmaline,  cyanite,  or  sillimanite,  in  mica 


schist  and  gneiss;  many  localities  are  known  in  New  Eng- 
land, very  perfect  crystals  are  found  in  Fanniii  County, 
Georgia,  and  in  North  Carolina. 

CHONDRODITE  is  a  silicate  of  magnesium  and  iron  con- 
taining fluorine.  It  occurs  in  yellow  grains  imbedded  in 
crystalline  limestone,  also  in  deep  red  crystals  associated, 
for  example,  with  magnetite,  as  at  Brewster,  N.  Y.  Hard- 
ness 6  to  6.5;  specific  gravity  3.1  to  3.2.  The  mineral 
HUMITE,  occurring  in  honey-yellow  crystals  at  Vesuvius 
(Mt.  Somma),  is  closely  related  to  chondrodite. 


Talc.  Magnesium  silicate, 
TALC  is  remarkable  among  minerals  because  of  its  soft- 
ness; on  this  account  it  is  placed  at  the  beginning  of  the 
scale  of  hardness.  It  is  easily  scratched  by  the  nail  and 
has  a  soapy,  unctuous  feel.  The  common  form  of  talc  is 
that  in  plates  or  leaves,  foliated  it  is  called,  which  separate 


DESCRIPTION    OF   MINERAL   SPECIES.  327 

easily  because  of  the  perfect  cleavage;  these  leaves  are  not 
elastic  like  those  of  mica,  but  flexible. 

There  are  also  kinds  of  talc  which  are  compact  and 
show  but  little  of  the  foliated  character;  this  is  especially 
true  of  the  kind  called  steatite  or  soapstone,  which  is 
sawed  into  slabs  and  used  for  hearths  and  furnaces,  also, 
when  pulverized,  as  a  lubricator.  The  Chinese  make  im- 
ages and  other  small  articles  out  of  a  fine-grained  com- 
pact kind,  and  a  similar  kind  is  used  for  slate-pencils. 
Another  variety,  derived,  however,  from  the  alteration  of  a 
different  mineral  (Enstatite,  p.  295),  is  fibrous,  a  little 
like  asbestus,  and  is  used,  when  ground  up,  for  giving  a 
silky  gloss  to  wall-paper  and  to  mix  with  wood-pulp  in 
making  paper;  it  is  obtained  from  Edwards,  N.  Y. 

The  hardness  of  talc  is  1,  as  stated  above,  and  its  specific 
gravity  is  2.8.  The  luster  is  pearly,  especially  in  the  foli- 
ated kinds,  and  the  color  in  the  finest  of  these  a  beautiful 
sea-green;  there  are  also  white  foliated  kinds,  and  the 
massive  varieties  may  be  dark  gray. 

Talc  is  a  silicate  of  magnesium,  with  the  formula 
H40.3Mg0.4SiO.  The  percentage  composition  is:  Silica 
(SiOJ  63.5,  magnesia  (MgO)  31.7,  water  (11,0)  4.8  =  100. 
Upon  being  heated  quite  hot  in  a  closed  tube  it  gives  off 
a  small  amount  of  water.  It  is  a  common  mineral,  often 
associated  with  serpentine  and  chlorite  rocks;  many  locali- 
ties are  known  in  the  eastern  United  States  and  Canada. 

Serpentine.     Magnesium  silicate,  H4Mg,Si,0, . 

SERPENTINE  is  a  remarkable  mineral  because  of  the 
variety  of  massive  forms  it  assumes,  although  it  is  not 


328  MINERALS,  AND   HOW  TO  STUDY   THEM. 

known  to  occur  in  crystals  of  its  own.  The  crystals  of 
serpentine  which  are  found  are  what  are  called  pseudo- 
morphs  (p.  55),  having  been  derived  from  some  other 
species  by  chemical  change.  Thus  the  magnesium-iron 
silicate  chrysolite — a  mineral  found  especially  in  basaltic 
rocks — is  often  changed  to  serpentine,  and  then  the  ser- 
pentine is  said  to  be  a  pseudomorph  after  the  chrysolite, 
since  it  retains  its  form. 

The  translucent  masses  of  serpentine  of  a  deep  oil- 
green  color  are  called  precious  serpentine.  A  clouded 
or  mottled  variety,  either  green  or  red,  is  called  ser- 
pentine marble  or  verd  -  antique  marble  or  ophiolite; 
this  last  name,  like  that  of  the  species,  refers  to  the 
serpent-like  markings  so  commonly  observed.  This  kind 
is  used  as  a  building  stone  in  Philadelphia  and  Bal- 
timore. Other  kinds  of  serpentine  are  foliated  or 
lamellar  and  separable  into 

oil). 

brittle  leaves.  The  most  pe- 
culiar variety  is  the  fine  fibrous 
kind  called  chrysotile  (not  to 
be  confounded  with  chrysolite). 
This  usually  occurs  as  thin 
seams  in  the  massive  mineral 
(Fig.  310).  Chrysotile  may  be 
separated  into  fibers,  very  flex- 
ible and  as  soft  as  the  finest  silk.  This  variety  is  popu- 
larly called  asbestus,  but  it  must  be  remembered  that 
there  is  another  kind  of  asbestus  of  rather  similar  appear- 
ance which  is  a  variety  of  the  mineral  amphibole  (p.  297). 
The  serpentine  asbestus  is  extensively  mined  in  the  prov- 


DESCRIPTION   OF   MINERAL   SPECIES.  329 

ince  of  Quebec,  as  at  Thetford,  where  it  occurs  in  seams 
sometimes  3  or  4  inches  in  thickness.  It  is  ground  up 
and  used  (since  it  is  a  non-conductor  of  heat)  for  packing 
steam-pipes,  etc.;  also  as  asbestus  roofing;  as  a  lubricator; 
and  so  on. 

The  hardness  of  serpentine  is  usually  from  2.5  to  3;  it 
is  hence  easily  scratched  and  often  has  a  smooth  feel, 
sometimes  almost  greasy;  some  varieties  are  harder  than 
3,  up  to  4  or  even  5.5.  The  specific  gravity  is  about  2.50. 
The  luster  is  usually  feeble  and  greasy  or  waxlike;  the 
color,  as  stated,  some  shade  of  green  to  gray  or  nearly 
white. 

The  composition  of  serpentine  is  given  by  the  formula 
3Mg0.2SiOa.2H90,  which  requires:  Silica  (SiOJ  44.1, 
magnesia  (MgO)  43.0,  water  (HaO)  12.9  =  100;  iron  is 
often  present  in  small  amount.  Heated  in  the  closed  tube 
it  yields  considerable  water,  but  it  fuses  in  the  forceps 
only  on  the  edges. 

Besides  the  two  minerals  talc  and  serpentine  there  is 
also  another  important  magnesium  silicate,  the  mineral 
SEPIOLITE,  better  known  as  meerschaum  (from  the  Ger- 
man). This  is  a  soft  white  mineral,  so  light,  because  of 
its  loose  texture,  as  to  nearly  float  on  water  when  quite  dry, 
and  this  is  what  gives  it  its  familiar  name,  which  means  sea- 
foam.  It  is  much  used  for  the  bowls  of  tobacco-pipes, 
and  for  this  purpose  is  mined  in  Asia  Minor. 

Datolite. 

DATOLITE  is  found  in  clear  glassy  crystals  having  usu- 
ally a  faint  green  tinge.  The  crystals  are  monoclinic  in 


330  MINERALS,  AND   HOW   TO   STUDY   THEM, 

crystallization,  and  are  complex  and  difficult  to  decipher 
even  for  one  who  has  had  a  good  deal  of  experience  (Fig. 
311,  also  Fig.  97,  p.  45).  It  is  noticed 
at  once,  however,  on  examining  the 
surface  spangled  with  crystals  that 
there  is  little  apparent  uniformity  in 
the  shape  of  the  faces,  which  is  in 
agreement  with  their  monoclinic  char- 
acter. 

There  is  also  a  kind  of  datolite  occur- 
ring in  forms  resembling  a  pinkish  porcelain,  but  it  is  only 
known  from  the  Lake  Superior  copper-mining  region;  also 
another  kind  in  botryoidal  masses;  but  both  of  these  are 
rare. 

The  hardness  is  5  to  5.5,  and  the  specific  gravity  about 
3.  The  luster  is  vitreous,  and  the  color  white  to  greenish 
or  pale  reddish. 

Datolite  is  a  silicate  of  boron  and  calcium.  It  yields  a 
little  water  when  heated  in  the  closed  tube,  and  gives  a 
green  flame  in  the  forceps  (boron),  at  the  same  time  fusing 


It  occurs  chiefly  in  the  kind  of  eruptive  rock  found  in 
New  Jersey,  Connecticut,  and  Massachusetts,  and  is  often 
associated  with  the  various  minerals  belonging  to  the  Zeo- 
lite family.  Like  them  it  is  a  secondary  mineral,  that  is, 
formed  after  the  rock  which  encloses  it,  and  usually  out  of 
the  chemical  material  the  rock  affords  by  partial  decom- 
position. 


DESCRIPTION   OF   MINERAL  SPECIES.  331 

Prehnite. 

PREHNITE  is  another  mineral  occurring  like  datolite 
under  the  same  conditions  as  the  zeolites  and  associated 
with  them.  It  is  seldom  in  distinct  crystals,  usually  in 
crystalline  masses  with  a  botryoidal  or  mammillary  surface 
(Fig.  138,  p.  68),  or  in  groups  of  tabular  crystals  showing 
a  series  of  little  ridges  in  parallel  position. 

The  hardness  is  6  to  6.5,  and  the  specific  gravity  about 
2.9.  The  luster  is  vitreous,  and  the  common  color  green, 
but  it  is  sometimes  white  or  gray. 

Prehnite  is  a  silicate  of  alumina  and  lime  yielding  a 
little  water  when  heated  in  the  closed  tube.  It  fuses  in 
the  forceps  rather  easily,  and  is  slowly  decomposed  by 
hydrochloric  acid. 

Apophyllite. 

APOPHYLLITE  stands  still  closer  to  the  Zeolites  than 
either  of  the  two  preceding  minerals;  it  is  not  only  re- 
lated to  them  in  the  way  it  occurs  and  in  association,  but 
also  in  its  chemical  composition — in  fact  it  is  sometimes 
called  a  zeolite. 

It  occurs  in  square  prismatic  or  pyramidal  crystals,  but 
of  a  considerable  variety  in  habit.  The  crystals  may  be 
simple  square  prisms  terminated  by  the  basal  plane  with 
its  characteristic  pearly  luster,  and  then  often  looking  like 
a  cube,  or  the  form  may  resemble  a  cube  whose  angles 
have  been  replaced,  like  Fig.  313  (compare  Fig.  107,  p. 
50).  Here  it  is  to  be  noted  that  the  angle  made  by 
the  pyramid  (p)  on  c  (119°  28')  is  not  the  same  as  that 
on  the  prism  a  (128°);  further,  it  is  noticed  on  close  ex- 


MINERALS,  AND   HOW   TO   STUDY   THEM. 


animation  that  the  faces,  a,  have  a  vitreous  luster  and 
are  often  striated  vertically,  while  the  base,  c,  has  a  pearly 
luster  and  is  often  dull.  Another  form  of  crystal  is 
sharply  terminated  by  the  pyramid  p,  and  still  another  is 
in  flat  tables  (Fig.  312). 

The  cleavage  is  perfect  parallel  to  the  top  or  base  of  the 
crystals  (c),  and  hence  the  peculiar  pearly  luster  noticed 


312. 


314. 


813. 


in  this  direction;  elsewhere  the  luster  is  vitreous.  The 
pearly  luster  is  so  peculiar  that  an  old  name  of  the  mineral 
(ichthyophthalmite)  means  fish-rye  *tone;  the  name  apo- 
phyllite  refers  to  the  character  of  exfoliating  before  the 
blowpipe.  The  hardness  is  4.5  to  5,  and  the  specific  gravity 
2.3  to  2.4.  The  crystals  may  be  entirely  colorless,  or  they 
may  be  white  or  rarely  of  a  beautiful  rose-pink. 

Apophyllite  is  a  silicate  containing  lime  and  potash  with 
a  good  deal  of  water  and  a  small  amount  of  fluorine.  Be- 
fore the  blowpipe  it  exfoliates,  whitens,  and  yields  acid 
water,  fusing  to  a  vesicular  enamel  and  coloring  the  flame 
a  violet  (potash).  Beautiful  specimens  have  been  obtained 
from  Bergen  Hill,  New  Jersey. 


DESCRIPTION   OF   MINERAL  SPECIES.  333 

Pectolite. 

PECTOLITE  is  often  found  with  prehnite,  datolite,  and 

various  of  the  zeolites.     The  common  form  is  massive  and 

radiated  or  stellate,  or  in  similar  forms  made  up  of  acicular 

crystals  (Fig.  315).     Distinct  crystals  are  rare;  these  are 

315. 


monoclinic  and  show  two  perfect  cleavages.  The  hardness 
is  5,  specific  gravity  2.68  to  2.78;  luster  silky  to  subvit- 
reous;  color  white  or  grayish. 

Pectolite  is  a  silicate  of  lime  and  soda  having  the  for- 
mula HNaCa2(Si03)3  or  HaO.Naa0.4Ca0.6Si09,  which  re- 
quires: Silica  (SiOs)  54.2,  lime  (CaO)  33.8,  soda  (Na.,0) 
9.3,  water  (HaO)  2.7  =  100.  It  yields  water  in  the  closed 
tube  and  fuses  easily  (at  2)  to  a  white  enamel.  It  is  de- 
composed by  hydrochloric  acid  with  separation  of  silica. 

THE   ZEOLITES. 

The  ZEOLITE  FAMILY  includes  a  number  of  beautiful 
minerals  having  a  close  relation  to  each  other  in  manner 


334  MINERALS,  AND   HOW  TO   STUDY  THEM. 

of  occurrence  and  in  their  chemical  composition.  They 
are  all  hydrous  silicates,  that  is,  they  contain  water,  which 
is  given  off  when  a  fragment  is  heated  in  a  closed  tube, 
and,  like  other  hydrous  silicates,  they  are  of  inferior  hard- 
ness, chiefly  3.5  to  5.5,  and  low  specific  gravity,  chiefly  2.0 
to  2.4.  They  are  readily  decomposed  by  hydrochloric 
acid,  some  of  them  forming  a  jelly.  Many  of  them 
bubble  up,  or  intumesce  when  heated  before  the  blowpipe, 
and  this  has  given  the  name  to  the  family  from  the  Greek 
(C,eiv)  to  boil. 

The  Zeolites  are  all  said  to  be  secondary  minerals,  which 
means  that  they  were  made  subsequent  to  the  time  of  for- 
mation of  the  rock  in  which  they  occur,  unlike  the  feld- 
spar, quartz,  etc.,  which  are  part  of  the  rock.  They  have 
been  formed  in  most  cases  out  of  the  materials  of  the  feld- 
spar or  related  minerals  in  the  rock  itself,  and  hence  occur 
rather  in  crevices,  seams,  or  in  cavities,  instead  of  in  the 
solid  mass. 

They  are  all  silicates  of  alumina,  and  with  this  lime  or 
soda  or  potash;  they  do  not  contain  iron  or  magnesia; 
hence  they  are  like  the  feldspar  and,  indeed,  are  often 
called  hydrous  feldspars. 

The  zeolites,  and  also  the  minerals  datolite,  prehnite, 
apophyllite,  pectolite  {also  calcite),  which  occur  with 
them,  are  found  frequently  in  connection  with  the  dark- 
colored  "trap  rock," such  as  that  which  forms  the  Palisades 
of  the  Hudson,  and  is  found  also  at  various  points  in  Con- 
necticut and  Massachusetts,  also  in  Nova  Scotia.  Famous 
localities  have  been  developed  where  railroad  cuts  or  tun- 
nels have  been  cut  through  ridges  of  this  and  similar 


DESCRIPTION   OF  MINERAL   SPECIES.  335 

igneous  rocks  (basalt,  etc.),  as  at  Bergen  Hill,  New  Jersey, 
also  similarly  in  British  India.  Beautiful  specimens  come 
from  Nova  Scotia.  The  zeolites  are  also  found,  but  not  so 
commonly,  in  granitic  rocks. 

Thomsonite. 

THOMSONITE  is  usually  found  in  columnar  masses  with 
radiated  structure,  also  in  radiated  spherical  concretions, 
as  in  the  beautiful  water-worn  pebbles  found  on  the  shores 
of  Lake  Superior  at  Grand  Marais.  Crystals  are  rare. 
The  hardness  is  5  to  5.5;  specific  gravity  2.3  to  2.4;  luster 
vitreous  or  slightly  pearly;  color  snow-white,  pale  red  or 
green.  It  is  a  silicate  of  alumina,  lime,  and  soda.  Before 
the  blowpipe  it  fuses  easily  (2)  with  intumescence  to  a 
white  enamel;  with  hydrochloric  acid  it  gelatinizes. 

Natrolite. 

NATROLITE  is  sometimes  called  the  needle  zeolite  because 
it  is  so  common  to  find  it  in  very  fine  acicular,  or  needle- 
like,  crystals.     These  crystals  are  often  arranged       316. 
in   radiating  tufts;    sometimes   they  line  oval 
cavities  in  the  enclosing  rock.     When  the  crys- 
tals are  larger,  they  are  seen  to  have  the  form  of 
a  nearly  square  prism  (angles  91°  15')  with  alow 
pyramid  on  the  summit  (Fig.  316).     There  are 
also  massive  varieties  having  a  fibrous  or  fine-columnar 
radiated  structure. 

The  hardness  is  5  to  5.5;  specific  gravity  2.2  to  2.25; 
luster  vitreous,  sometimes  a  little  pearly;  colorless  or 
white,  also  gray,  yellowish,  and  reddish.  It  is  a  silicate  of 
alumina  and  soda  (natron),  as  its  name  indicates.  Before 


336  MINERALS,  AND    HOW   TO   STUDY   THEM. 

the  blowpipe  it  fuses  quietly  at  2  to  a  colorless  glass;  with 
hydrochloric  acid  it  gelatinizes. 

SCOLECITE  is  a  rarer  zeolite  resembling  natrolite  in  its 
massive  forms,  but  it  contains  lime  instead  of  soda.  It 
takes  its  name  from  its  behavior  before  the  blowpipe, 
curling  up  as  a  worm  before  fusion. 

Analcite. 

ANALCITE  is  found  in  crystals  having  usually  the  form 
of  a  trapezohedron,  shown  in  Fig.  317,  resembling  one  of 
the  common  forms  of  garnet.  It  also  less 
frequently  occurs  in  cubes  with  three  faces 
on  each  solid  angle,  like  Fig.  16,  page  24. 
The  hardness  is  5  to  5.5;  specific  gravity 
2.22  to  2.3;  luster  vitreous;  colorless,  white, 
or  pale  yellow  or  gray.  It  fuses  before  the 
blowpipe  at  2.5  to  a  colorless  glass  and  gelatinizes  with 
hydrochloric  acid. 

Chabazite. 

CHABAZITE  is  found  in  rhombohedrons,  but,  as  the  angle 
between  two  faces  (95°)  is  not  far  from  90°,  their  aspect 
is  often  that  of  cubes,  and  it  is  possible  318. 

for  one  not  experienced  to  make  a  mistake. 
The  crystals  often  interpenetrate  each  other 
(Fig.  318)  in  twining  position  a  little  like 
the  cubes  of  fluorite. 

The  hardness  is  4  to  5;  specific  gravity  2.08  to  2.16, 
Luster  vitreous;  color  white,  yellow,  also  flesh-red.  In 
composition  it  is  a  silicate  of  alumina  and  lime  with  small 
amounts  of  soda  and  potash.  Before  the  blowpipe  it  intu- 


DESCRIPTION   OF  MINERAL  SPECIES.  337 

mesces  and  fuses  to  a  blebby,  nearly  opaque  glass;  it  is 
decomposed  by  hydrochloric  acid  with  the  separation  of 
slimy  silica. 

Acadiullte  is  a  reddish  variety  of  chabazite  from  Nova 
Scotia;  pliceolite  a  colorless  kind  from  Bohemia;  and  hay- 
denite  a  yellowish  kind  from  Jones  Falls  near  Baltimore. 

GMELINITE  is  another  rhombohedral  zeolite  closely  re- 
lated to  chabazite,  but  much  rarer. 

Stilbite. 

STILBITE  is  named  from  one  of  its  most  important  char- 
acters, its  beautiful  pearly  luster  on  the  side  cleavage  face. 
319      It  is  usually  found  in  bundles  of  crystals,  often 
looking  like  a  sheaf  of  wheat  tied  tightly  about 
the  centre  (Fig.  319);  this  has  given  it  another  of 
its  names,  desmine,  from  the  Greek  (dea-fiios)  for 
bundle.     There  are  also  radiated  forms,  seen,  for 
example,  on  a  flat   surface  of  rock,  but  distinct 
crystals  are  rare.     The  cleavage  is  very  perfect  on  the  side 
face. 

The  hardness  is  3.5  to  4,  and  the  specific  gravity  2.1  to 
2.2.  The  luster,  as  already  stated,  is  brilliant  pearly  on 
the  cleavage  surface,  the  side  face  of  the  bundles  which 
have  been  described.  The  color  varies  from  white  to  yel- 
low, and  also  to  red  or  brown. 

Stilbite  is  a  silicate  of  alumina  and  lime.  Before  the 
blowpipe  it  exfoliates,  swells  up,  and  fuses  (2  to  2.5)  to  a 
white  enamel. 

HARMOTOME  is  a  rather  rare  zeolite,  remarkable  for  con- 
taining 20  per  cent  of  baryta.  It  is  usually  found  in  white 


338  MINERALS,  AND   HOW   TO   STUDY   THEM. 

to  yellow  or  brown  crystals  of  complex  twinned  structure. 
PHILLIPSITE  is  a  related  species,  not,  however,  containing 
baryta;  the  crystals  are  similar  to  those  of  harmotome, 
but  the  color  does  not  often  vary  from  white. 

Heulandite. 

HEULANDITE,  named  after  the  owner  of  the  famous 
Heuland  cabinet,  is  found  in  fine  monoclinic  crystals  with 
perfect  cleavage  parallel  to  the  side  plane,  on  which  it  has 
also  a  marked  pearly  luster  that  is  very  characteristic.  It 
is  like  stilbite  in  this  respect,  but  very  different  in  form. 
The  hardness  is  3.5  to  4,  and  the  specific  gravity  2.2.  The 
color  is  usually  white,  often  milk-white,  but  red,  gray,  or 
brown  varieties  also  occnr.  It  resembles  gypsum  a  little, 
but  is  much  harder. 

LAUMONTITE  is  a  lime  zeolite  containing  a  large  amount 
of  water,  part  of  which  it  is  apt  to  lose  on  exposure  to  a 
dry  air,  whence  it  frequently  falls  to  pieces  in  the  cabinet. 
The  common  color  is  white  (also  red) ;  the  form  mono- 
clinic. 


ON  THE  DETERMINATION  OF  MINERALS.  339 

CHAPTER  VIII. 
ON  THE  DETERMINATION  OF  MINERALS. 

IT  will  seem  to  the  beginner  a  difficult  thing  to  become 
so  familiar  with  the  many  different  mineral  species  as  to 
be  able  to  recognize  each  of  them  at  sight;  and  it  is  diffi- 
cult, in  fact  impossible,  even  for  the  trained  mineralogist 
to  be  always  prompt  and  sure  in  his  determination.  For 
there  are  a  large  number  of  distinct  species,  between  800 
and  1000,  many  of  them  very  rare,  while  not  a  few  appear 
in  a  great  variety  of  forms.  In  the  latter  case  the  varie- 
ties sometimes  depend  upon  fundamental  differences  of 
chemical  composition,  as  among  the  garnets;  and  some- 
times upon  less  essential  distinctions  of  structure,  color, 
and  so  on,  as  with  the  varieties  of  quartz,  calcite,  and 
fluorite.  Hence  it  is  obvious  that  the  characters  that  can 
be  perceived  at  once,  without  the  aid  of  careful  tests,  are 
often  insufficient  to  fix  a  mineral  positively.  The  experi- 
enced mineralogist,  while  he  learns  to  know  minerals  so 
well  that  he  can  name  most  of  them  at  sight  and  seldom 
blunders,  is  ever  distrustful  of  himself  and  often  hesitates 
to  give  the  name  quartz  to  a  specimen  having  to  the  eye 
the  external  characters  of  this  common  species  without, 
for  example,  a  confirmatory  test  of  hardness.  Confidence 
and  hasty  judgment  belong  to  those  who  have  little  ex- 
perience and  a  scanty  knowledge  of  the  difficulties  of  the 
subject. 


340  MINERALS,  AND   HOW  TO   STUDY   THEM. 

But,  on  the  other  hand,  to  recognize  most  of  the  min- 
erals, which  are  likely  to  be  collected  on  a  mineralogical 
excursion  or  to  be  obtained  by  exchange  with  other  col- 
lectors, is  generally  easy  even  for  the  beginner,  if  he  goes  at 
the  subject  in  the  right  way.  For  the  number  of  common 
species  is  small,  and  quartz,  feldspar,  mica,  calcite,  and 
barite,  also  galena,  sphalerite,  pyrite,  chalcopyrite,  among 
metallic  species,  are  constantly  presenting  themselves,  and 
though  their  characters  vary  somewhat  widely  in  different 
specimens,  these  are  usually  distinct,  and  almost  always  a 
simple  test  will  make  the  matter  sure. 

First  of  all,  then,  the  mineralogist  should  know  these 
and  other  common  species  well,  for  the  chances  are  many 
times  greater  that  an  unknown  specimen  is  one  of  them 
than  that  it  is  a  rare  and  little-known  species.  It  may  be 
rare,  even  a  new  one  not  before  described  and  not  given  in 
any  of  the  books;  but  this  is  a  chance  that  does  not  often 
happen.  A  real  difficulty,  that  even  much  experience 
does  not  entirely  remove,  lies  in  the  fact  that  at  any  large 
mineral  locality  there  are  likely  to  be  many  nondescript 
specimens  which  show  few  distinct  characters.  Some- 
times these  are  mixtures  of  several  species,  and  often  they 
arise  from  chemical  decomposition  of  well-known  min- 
erals. About  such  specimens  it  may  perhaps  be  impos- 
sible to  say  anything  definite;  in  fact,  exhaustive  micro- 
scopic and  chemical  work  is  often  needed  to  settle  their 
character.  In  such  cases  the  beginner  may  well  turn  to 
some  one  more  experienced  for  counsel. 

The  best  way,  then,  for  one  with  a  specimen  of  an  un- 
known mineral  in  hand  is  to  think  of  the  common  species 


ON   THE    DETERMINATION   OF   MINERALS.  341 

first,  and  afterward  of  others  which  may  suggest  them- 
selves, running  over  in  mind  or  by  reference  to  the  book 
the  characters  observed  and  those  pf  the  species  to  which 
it  is  provisionally  referred,  but  with  care  not  to  decide  too 
hastily,  but  to  give  each  character  full  weight.  Do  not 
give  the  name  albite  to  a  specimen  of  barite,  either  the 
tabular  glassy  crystals  or  the  white  massive  granular  kind, 
because  both  species  are  often  white  and  also  resemble 
each  other  in  form,  and  overlook  the  fact  that  it  is  much 
too  heavy  as  well  as  too  soft.  Do  not  give  the  name  beryl 
to  a  crystal  of  apatite  because  it  is  a  green  hexagonal 
prism,  and  overlook  the  fact  that  it  is  quite  too  hard. 
Finally,  do  not  hesitate  to  confess  ignorance — that  the  ex- 
perienced mineralogist  is  ever  ready  to  do,  and  it  is  thia 
fact  that  enables  him  from  time  to  time  to  identify  some 
rare  and  interesting  species  and  perhaps  occasionally  one 
new  to  science. 

As  the  student  goes  on  to  learn  minerals  better  and  bet- 
ter,the  knowledge  of  the  commoner  species  becomes  so  im- 
pressed upon  his  mind  that  he  seldom  hesitates,  when  a 
specimen  of  one  of  them  is  put  in  his  hands,  but  its  name 
suggests  itself  at  once,  and  this  without  the  careful  sum- 
mary and  comparison  of  characters  which  the  beginner 
must  go  through.  Then  a  confirmatory  test  clinches  the 
matter.  But  this  power  only  comes  with  long  experience, 
and  even  then  it  is  sometimes  necessary  to  carry  on  an 
exhaustive  examination  before  a  result  is  reached. 

In  the  systematic  determination  of  an  unknown  speci- 
men the  first  thing  to  do,  as  was  insisted  upon  in  Chapter 
II,  is  to  learn  all  that  is  possible  about  it  by  looking  at  and 


342  MINERALS,    AND   HOW   TO   STUDY   THEM. 

handling  it.  It  has  already  been  shown  that  in  this  way 
its  form  and  structure  may  be  at  least  partially  deter- 
mined; also  its  cleavage,  if  it  shows  any;  and  finally  its 
luster,  color,  degree  of  transparency,  etc.  But  at  the 
same  time  the  other  senses  must  be  kept  on  the  alert,  so 
that,  for  example,  if  the  specimen  is  particularly  heavy 
or  light,  greasy  to  the  touch,  etc.,  all  these  points  will  be 
quickly  perceived  and  duly  regarded. 

Then  a  touch  with  the  point  of  a  knife-blade  will  show 
something  as  to  the  hardness.  This,  it  must  be  repeated, 
should  be  done  carefully  so  as  not  to  spoil  the  specimen, 
and  the  student  must  be  on  his  guard  not  to  make  any 
of  the  mistakes  easily  possible  in  such  trials,  as  before 
pointed  out  (pages  76, 77).  If  the  mineral  is  not  scratched 
by  the  knife  it  will  be  well  to  see  whether  it  will  scratch  a 
piece  of  window-glass,  and  then  whether  it  is  scratched  by 
or  will  scratch  the  smooth  surface  of  a  quartz  crystal — for 
the  number  of  minerals  as  hard  or  harder  than  quartz  is 
very  small,  as  further  stated  on  p.  357. 

At  the  same  time  with  the  test  for  hardness,  the  streak, 
or  color  of  the  powder,  left  by  the  knife  or  on  a  surface  of 
ground  glass  or  unglazed  porcelain,  must  be  noticed.  Also 
if,  as  is  most  desirable,  the  blade  of  the  knife  is  a  magnet, 
the  distinguishing  character  of  magnetite  and  pyrrhotite 
will  show  itself  at  once.  The  careful  determination  of 
the  specific  gravity  requires  more  time,  and  may  be  post- 
poned till  the  blowpipe  has  been  used,  but  the  hand  should 
have  already  made  a  rough  estimate  of  this,  as  has  been 
before  remarked  (p.  79  et  seq.). 

Further,  when  the  characters  mentioned  have  all  been 


ON    THE    DETERMINATION    OF   MINERALS.  343 

noted  it  Will  be  often  necessary  to  make  some  chemical 
tests  (read  carefully  pp.  153  to  157).  A  fragment  for 
examination  can  generally  be  obtained  by  a  careful  blow 
with  a  light  hammer  without  injury  to  the  specimen.  This, 
placed  in  a  test-tube  or  on  a  watch-glass,  with  a  little  strong 
hydrochloric  (or  nitric)  acid  (p.  155),  will  effervesce  with  a 
nearly  odorless  gas  (carbon  dioxide,  CO,)  if  it  is  a  carbonate. 
Calcite  effervesces  at  once,  even  in  large  fragments,  in  dilute 
acid,  and  other  carbonates  will  act  in  the  same  way  in  strong 
acid,  and  also  in  dilute  if  they  are  first  pulverized  or  the 
tube  is  warmed  (p.  155).  But  remember  that  sulphureted 
hydrogen  or  hydrogen  sulphide  (H3S)  may  be  liberated 
from  a  sulphide  by  warm  hydrochloric  acid  (p.  155),  and 
do  not  decide  a  given  specimen  is  siderite  instead  of  spha- 
lerite hastily  because  effervescence  is  observed,  and  over- 
look the  strong  offensive  odor  of  the  hydrogen  sulphide; 
in  other  words,  at  all  times  the  different  senses  should  act 
together. 

The  solution  obtained  will  give  the  chemist  the  means  of 
learning  more  (e.g.,  the  presence  of  copper,  p.  156),  and  if 
the  specimen  is  insoluble,  even  when  finely  powdered  and 
heated  in  acid,  that  is  an  important  point  (pp.  156,  157). 

The  blowpipe  tests  may  come  before  or  after  the  other 
chemical  examination,  and  these  have  been  so  fully  ex- 
plained in  Chapter  VI  that  they  need  not  be  repeated 
here.  A  careful  study  of  this  chapter  will  have  given  the 
student  full  command  of  this  part  of  the  subject,  and  his 
experience  should  have  taught  him  what  order  is  best  for 
the  different  tests.  He  will  have  learned,  for  example, 
that  a  mineral  with  metallic  luster  should  first  be  tried  in 


344  MINERALS,  AND    HOW   TO   STUDY    THEM. 

the  open  tube.  If  sulphur  is  present  (the  mineral  being  a 
sulphide)  it  will  be  given  off  as  sulphur  dioxide  (pp.  149, 
150),  and  at  the  same  time  arsenic,  antimony,  and  mercury 
will  show  themselves  (p.  148  et  seq.}.  The  closed  tube 
may  be  taken  next  and  then  the  charcoal  (p.  140),  which 
last  will  confirm  the  results  already  obtained,  and  also 
show  by  the  coating  the  presence  of  zinc,  lead,  etc. ;  also, 
by  adding  soda  in  most  cases,  the  presence  of  a  reducible 
metal  (p.  139)  may  be  proved,  as  lead,  silver,  tin;  while  a 
magnetic  residue  will  indicate  iron.  Further,  after  roast- 
ing off  (p.  139)  the  sulphur,  arsenic,  or  antimony  the 
residue  may  be  tested  for  copper,  cobalt,  etc.,  with  borax 
on  the  platinum  wire. 

A  mineral  with  unmetallic  luster  may  be  tried  first  in 
the  forceps  (but  always  with  caution :  it  may  contain  anti- 
mony, for  example),  and  the  degree  of  fusibility,  the  flame 
coloration,  and  other  phenomena  noted  (p.  130,  et  seq.}, 
and  then  an  examination  made  on  the  platinum  wire 
(p.  136  et  seq.},  in  the  bead  of  borax  or  salt  of  phosphorus. 
In  the  latter  a  silicate  leaves  a  skeleton  of  undissolved 
silica  having  the  form  of  the  individual  fragment  (p.  140). 

Note,  finally,  that  to  obtain  correct  and  concordant 
results  the  pure  mineral  must  be  experimented  upon.  In 
many  specimens  two  or  more  species  are  so  closely  mixed 
together  that  it  needs  sharp  eyes,  aided  by  a  magnifying- 
glass,*  to  separate  them;  this  is  particularly  true  of  metal- 
lic minerals.  Also  many  species  commonly  occur  in  an 

*  Every  mineralogist  should  have  a  pocket  magnifying-glass,  for 
even  good  eyes  often  need  assistance,  especially  in  examining  small 
crystals. 


ON    THE   DETERMINATION  OF   MINERALS.  345 

earthy  mass,  or  gangue,  so  that  it  is  difficult  to  obtain 
absolutely  pure  material.  In  such  cases  the  quartz  or  clay 
will  often  do  no  harm  if  its  presence  is  noted  and  the 
results  interpreted  correctly.  A  fragment  of  cinnabar  is 
entirely  volatile  on  charcoal  or  in  the  tube,  but  frequently 
it  is  associated  with  a  gangue  of  clay,  and  then  this  will  of 
course  be  left  behind;  also  in  such  cases  a  fragment  heated 
in  the  glass  tube  often  yields  water  which  comes  from  the 
nonessential  gangue. 

Even  if  at  the  commencement  it  seemed  as  if  very  little 
was  known  about  a  specimen,  the  careful  use  of  the  eyes, 
the  hand,  and  the  various  tests  which  may  be  made  in  a 
few  minutes  will  have  given  a  pretty  complete  table  of  its 
characters,  and  these  may  be  used  to  fill  out  the  blank  list 
as  suggested  on  page  160.  In  most  cases,  unless  the  speci- 
men is  quite  rare  and  unusual,  it  will  be  possible  to  suggest 
the  name  of  a  species  with  the  description  of  which  it  is  to 
be  compared.  Where  this  method  of  attack  yields  no  defi- 
nite result  complete  determination  tables*  may  be  em' 
ployed,  and  in  the  hands  of  one  who  is  skillful  in  the  use  of 
the  blowpipe  and  in  the  simple  chemical  tests  they  will  quite 
surely  make  it  possible  to  identify  any  distinct  species. 

In  order  to  facilitate  the  work  of  determination,  and  at 
the  same  time  to  emphasize  prominent  characters  of  many 
minerals,  the  following  notes  are  given.  It  is  intended,  as 
a  rule,  to  mention  only  the  prominent  species  under  each 
head  and  those  showing  the  given  characters  most  dis- 
tinctly; to  enumerate  all  which  might  be  fairly  included 
would  deprive  the  lists  of  their  value. 

*  As  those  given  in  Brush's  Determinative  Mineralogy. 


346  MINERALS,  AND  HOW  TO  STUDY  THEM. 

1.  CRYSTALLINE  FORM. 

Cubes. — Fluorite  (p.  245)  is  the  common  mineral,  with 
unmetallic  luster,  which  is  likely  to  occur  in  cubes;  it  is 
easily  recognized  further  by  its  octahedral  cleavage. 

Halite  (p.  268),  or  rock  salt,  also  occurs  in  cubes,  but  has 
cubic  cleavage  and  its  slightly  sticky  feel  makes  it  natural 
to  test  its  taste,  which  at  once  removes  all  doubt. 

Pharmocosiderite  (p.  225)  is  a  rare  arsenate  of  iron 
which  also  occurs  in  yellow  or  greenish  cubes;  the  blow- 
pipe (e.g.,  on  charcoal)  shows  the  presence  of  both  iron 
and  arsenic. 

Galena  (p.  198),  of  metallic  minerals,  is  frequently  in 
cubes,  and  is  easily  recognized  by  its  cubic  cleavage,  high 
specific  gravity,  and  lead-blue  color. 

Pyrite  (p.  213)  in  cubes  is  known  by  its  light  brass-yel- 
low color,  brilliant  metallic  luster,  and  hardness;  further- 
more, the  cubes  usually  show  fine  lines  or  striations  parallel 
to  one  pair  of  edges  (see  Fig.  183,  p.  213). 

There  are  also  some  minerals  crystallizing  in  other  forms 
nearly  like  the  cube. 

Apophyllite  (p.  331)  may  have  a  form  resembling  a 
cube,  though  it  is  really  a  square  prism.  This  is  distin- 
guished by  the  pearly  luster  on  one  face,  parallel  to  which 
there  is  easy  cleavage,  while  the  four  other  faces  show  fine 
lines  or  striations  in  one  direction. 

Chabazite  (p.  336)  is  often  in  rhombohedrons  not  far  from 
a  cube  in  angle.  Calcite  (p.  247)  too,  though  the  common 
rhombohedron  cannot  be  mistaken  for  a  cube,  occasionally 
takes  a  form  very  near  it  in  angle.  This  form  has  been 


ON   THE   DETERMINATION   OF   MINERALS.  347 

called  a  cuboid.  Even  quartz  (p.  273)  appears,  though 
rarely,  in  forms  resembling  cubes  when  the  fundamental 
rhombohedron  is  present  almost  alone;  the  same  can  be 
said  of  hematite  (Fig.  192,  p.  218).  A  rare  sulphate  of  alu- 
minium, called  alunite  (p.  244)  (it  becomes  blue  after 
ignition  if  moistened  with  cobalt  solution)  also  occurs  in 
cubelike  rhombohedrons. 

Cryolite  (p.  242),  though  crystallizing  in  the  monoclinic 
system,  has  often  a  form  deceptively  like  a  cube  to  the  eye, 
and  the  actual  variation  from  this  in  angle  is  not  great;  it 
is  recognized  by  its  ready  fusibility  and  bright  yellow 
flame  (sodium). 

Octahedrons. — Fluorite  (p.  245)  sometimes  occurs  in  octa- 
hedrons, though  the  cube  is  much  more  common. 

Cuprite  (p.  194)  often  takes  this  form  and  is  at  once  told 
by  its  red  color  and  streak.  Spinel  (p.  241 )  is  another  octa- 
hedral mineral  remarkable  for  its  hardness.  Alum  and 
diamond  (p.  166)  are  common  in  octahedrons,  but  each  has 
other  characters  by  which  it  is  readily  recognized. 

Among  METALLIC  species  magnetite  (p.  219)  and  frank- 
Unite  (p.  221)  are  also  often  in  black  octahedrons;  but 
though  they  look  alike,  the  former  is  strongly  magnetic, 
the  latter  only  very  feebly  so  if  at  all ;  the  former  has  a 
black  streak,  while  that  of  the  latter  is  brown.  Chromite 
(p.  221)  is  another  mineral  sometimes  found  in  black  octa- 
hedrons. Pyrite  (p.  213)  is  often  in  brass-yellow  octa- 
hedrons, and  chalcopyrite  (p.  191)  sometimes  appears  in 
forms  resembling  them.  Galena  (p.  198)  occasionally 
occurs  in  octahedrons. 

A  number  of  minerals  occur  in  square  pyramids  looking 


348  MINERALS,  AND   HOW  TO   STUDY   THEM. 

more  i,r  less  like  regular  octahedrons,  as  noted  below,  and 
occasionally  a  rhombohedral  mineral  resembles  one;  this 
is  true,  for  example,  of  some  dark  colored  dolomite  crys- 
tals from  Spain. 

Dodecahedrons. — Garnet  (p.  300)  is  frequently  in  dodeca- 
hedral  crystals,  which  are  hard  (unless  altered  on  the  out- 
side), and  commonly  dark  red  to  black  in  color.  Magnetite 
(p.  219)  also  occurs  in  black  magnetic  dodecahedrons,  and 
sometimes  cuprite  (p.  194)  has  this  form. 

It  is  to  be  remembered  that,  even  if  the  whole  crystal 
cannot  be  seen,  the  diamond  face  with  angles  of  60° 
and  120°  is  very  characteristic  and  rarely  belongs  to  any 
other  crystalline  form;  moreover  the  angle  between  two 
adjacent  faces  of  a  dodecahedron  is  always  120°. 

Trapezohedrons.—  Garnet  (p.  300)  is  also  often  in  trape- 
zohedrons;  it  is  easily  distinguished  from  analcite  (p.  330), 
which  may  have  the  same  form,  but  whose  hardness  is  only 
5-5.5.  Further,  the  rarer  mineral  leucite  (p.  291),  found 
in  Vesuvian  lavas,  is  also  trapezohedral. 

Do  not  fail. to  notice  that  the  quadrilateral  face  of  this 
form  is  very  characteristic  and  will  often  make  it  possible 
to  recognize  it  when  only  a  small  part  of  a  crystal  is  visible. 

Pyritohedrons. — The  pyritohedral  form  is  characteristic 
of  pyrite  (p.  213),  known  also  by  its  pale  brass-yellow 
color.  Cobaltite  (p.  228)  has  the  same  form,  but  it  is  a 
rare  mineral  with  a  tin-white  color;  it  is  an  arsenide  of 
cobalt. 

Tetrahedrons. — Among  minerals  with  unmetallic  luster 
the  tetrahedron  is  common  with  zinc  blende  or  sphalerite 
(p.  233)  and  the  rare  minerals  boracite  (p.  262)  and  helvite 


ON   THE    DETERMINATION   OF   MINERALS.  349 

(p.  303).  This  is  also  true  of  tetrahedrite  (p.  193\  recog- 
nized by  its  black  color  and  brilliant  metallic  luster. 

Further,  chalcopyrite  (p.  191)  not  infrequently  occurs 
in  forms  called  sphenoids,  which  resemble  a  tetrahedron 
closely  and  only  differ  a  little  from  it  in  angles. 

Square  pyramids  are  common  with  zircon  (p.  ;>12)  and 
vesuvianite  (p.  315),  also  xenotime  (p.  271),  octahedrite 
(Fig.  43,  p.  32),  wulfenite  (p.  203),  and  a  few  other  rare 
species.  Some  of  these  forms  look  a  little  like  a  regular 
octahedron. 

Square  prisms  are  common  with  zircon  (p.  312),  vesu- 
viauite  (p.  315),  apophyllite  (p.  331),  scapolite  (p.  314). 
Square  tables,  often  very  thin,  are  characteristic  particu- 
larly of  wulfenite  (p.  203) ;  they  also  occur  with  apophyl- 
lite. 

Hexagonal  Pyramids. — Quartz  (p.  273)  is  the  mineral 
most  often  found  in  hexagonal  pyramids,  but  these  usually 
show  the  planes  of  the  hexagonal  prism  also  (see  figures  on 
pp.  273,  274),  and  often  one  set  of  three  alternate  planes 
at  one  end  are  larger  than  the  other  set,  as  explained  on 
the  pages  referred  to.  What  appears  to  be  the  same  form 
belongs  to  witherite  (p.  265),  but  it  is  really  a  compound 
twinning  form;  it  is  easily  distinguished  by  its  softness. 

Corundum  (p.  239)  also  has  this  form,  but  is  recognized 
by  its  hardness  and  adamantine  luster.  A  scalenohedron 
sometimes  looks  like  a  hexagonal  pyramid,  but  is  easily 
distinguished,  as  noted  below. 

Hexagonal  Prisms. — Beryl  (p.  298)  is  often  in  hexagonal 
prisms;  the  color  is  usually  green  (also  blue  and  yellow), 
and  it  is  hard  (H.  =  7-7.5).  Apatite  (p.  254)  has  nearly 


350  MINERALS,  AND   HOW   TO   STUDY   THEM. 

the  same  form  and  color,  but  is  soft  enough  to  be  scratched 
by  the  knife  (H.  =  5).  The  prisms  of  apatite  are  often 
terminated  by  the  planes  of  a  hexagonal  pyramid ;  this  is  less 
often  observed  with  beryl.  Pyromorphite,  mimetite,  and  van- 
adinite  (pp.  200,  201)  are  also  found  in  small  hexagonal 
prisms,  but  the  crystals  are  often  bundled  together  and 
not  seldom  hollow  or  cavernous. 

Quartz  (p.  273)  is  often  in  hexagonal  prisms,  and  these 
commonly  show  fine  horizontal  lines  or  striations.  This 
form  is  also  shown  by  the  different  species  of  mica  (p. 
303),  but  they  have  other  distinctive  characters. 

Calcite  (p.  247)  has  often  this  form,  sometimes  long 
prisms,  or  again  short  six-sided  tables;  it  is  soft  (H.  =  3) 
and  usually  has  its  terminal  planes  in  threes;  even  if  it 
has  only  a  single  flat  basal  plane,  the  cleavage  on  three 
alternate  edges  of  the  prism  is  characteristic. 

Tourmaline  (p.  317)  is  often  in  hexagonal  prisms  with 
three  rhombohedral  faces  at  each  end;  three  sided  and 
nine-sided  prisms  are  also  common  with  tourmaline.  The 
usual  color  is  black. 

Willemite  (p.  236),  the  silicate  of  zinc,  also  appears  in 
hexagonal  prisms. 

Trigonal  Prisms. — This  form  is  characteristic  of  tourma- 
line (p.  317);  if  the  crystals  are  imbedded  in  the  rock,  some 
of  the  cross-sections  made  by  fracture  will  usually  show 
the  shape  of  an  equilateral  triangle. 

It  must  be  noted  that  an  isometric  dodecahedron,  placed 
with  the  line  joining  the  two  trihedral  solid  angles  (those 
formed  by  three  adjoining  planes)  vertical,  has  the  form  of 
a  hexagonal  prism  with  three  rhombohedral  faces  at  each 


ON  THE   DETERMINATION   OF   MINERALS.  351 

end,  also  meeting  at  angles  of  120°  (compare  Figs.  104, 
105,  p.  49). 

Aragonite  (p.  252)  has  a  form  which  is  often  an  appa- 
rent hexagonal  prism,  but  it  is  really  due  to  twinning. 

Rhombohedrons. — A  rhombohedral  form,  with  cleavage 
parallel  to  faces  making  angles  of  105°  to  107°  with  each 
other,  is  characteristic  of  calcite  (p.  247)  first  of  all,  also 
dolomite  (p.  260),  siderite  (p.  223),  rhodochrosite  (p.  232). 
Smithsonite  (p.  237)  belongs  to  the  same  group,  but  is 
seldom  in  distinct  crystals. 

Chabazite  (p.  336)  is  also  often  in  rhombohedral  crystals, 
but  they  are  near  a  cube  in  angle,  and  it  shows  no  distinct 
cleavage.  The  same  is  true  of  alunite. 

Scalenohedrons. — Calcite  crystals  (p.  247)  are  not  in- 
frequently complete  scalenohedrons  (dog-tooth  spar},  or 
the  form  may  be  a  hexagonal  prism  with  scalenohedral 
faces  at  one  end.  A  scalenohedron,  if  complete,  is  at 
once  distinguished  from  a  hexagonal  pyramid  by  its  zig- 
zag basal  edge,  or  in  any  case  by  the  fact  that  the  angle 
of  one  set  of  three  alternate  terminal  edges  is  greater 
than  that  of  the  other  set. 

Rhombic  Prisms. — The  following  minerals,  among  the 
many  orthorhombic  species,  often  show  a  distinct  pris- 
matic habit :  topaz  (p.  320),  staurolite  (p.  325)  ;  also 
barite  (p.  262)  and  celestite  (p.  266). 

The  same  is  true  of  the  following  monoclinic  species: 
pyroxene  (p.  292),  orthoclase  (p.  285).  More  or  less  slender 
crystals  of  prismatic  form  or  aspect  occur  often  with  the 
varieties  of  amphibole  (p.  296)  and  with  epidote  (p.  316). 
Other  species  might  be  included  here. 


352  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Tabular  Crystals.  —  Crystals  flattened  parallel  to  one 
pair  of  faces  are  common  with  barite  (p.  262);  they  are 
often  clustered  in  divergent  groups.  This  is  also  true  of 
celestite  (p.  266).  Something  of  a  similar  form  is  seen 
with  crystals  of  albite  (p.  288). 

Acicular  Crystals. — Very  slender,  needlelike  crystals, 
often  in  radiating  groups,  are  characteristic  especially  of 
some  of  the  zeolites,  as  natrolite  (p.  335)  and  the  carbon- 
ate, aragonite  (p.  252).  Both  are  white,  but  the  latter 
effervesces  in  hydrochloric  acid.  Cuprite  (p.  194)  some- 
times appears  in  bright  red  capillary  crystals.  Of  metal- 
lic species,  stibnite  (p.  176)  often  occurs  in  groups  of 
radiating  acicular  crystals.  Jamesonite  (p.  200),  a  rare 
sulphide  of  antimony  and  lead,  looking  much  like  stibnite, 
also  occurs  occasionally  in  capillary  crystals. 

Millerite  (p.  227)  is  found  in  small  radiating  tufts  of 
slender  crystals  and  in  capillary  forms  resembling  a  bunch 
of  stiff  hairs. 

STRUCTURE. 

Fibrous. — (a)  With  Separable  Fibers. — Asbestus,  a  variety 
of  amphibole  (p.  296),  and  chrysotile  (often  also  called 
asbestus),  a  variety  of  serpentine  (p.  328),  belong  here. 
The  latter  contains  considerable  water  and  is  more  silky 
than  the  other  species.  This  fibrous  character  also  belongs 
to  a  rare  mineral  called  crocidolite  (p.  298),  which  in  its 
unaltered  form  has  a  bright  blue  color. 

(b)  Fibers  Not  Separable. — Of  the  many  species  hav- 
ing fibrous  varieties  the  most  important  are  calcite  and 
gypsum,  each  of  which  has  a  variety  called  satin  spar  j 


ON  THE    DETERMINATION    OF   MINERALS.  353 

also  aragonite,  barite,  celestite,  anhydrite,  brucite,  wavel- 
lite. 

The  following  are  more  commonly  COLUMNAR  rather 
than  fibrous:  amphibole,  epidote  (and  zoisite),  sillimanite, 
tourmaline,  natrolite,  and  several  other  zeolites ;  also 
strontianite  and  witherite.  Cyanite  is  bladed  rather  than 
columnar. 

Of  metallic  species,  stibnite  is  the  most  conspicuous 
example  of  columnar  structure. 

Radiated. — A  radiated  and  more  or  less  fibrous  struc- 
ture in  massive  varieties  is  seen  conspicuously  in  some 
varieties  of  the  following  species  :  natrolite  (p.  335), 
thomsonite  (p.  335),  stilbite  (p.  337);  also  amphibole  (p. 
296),  wavellite  (p.  243,  also  Fig.  133,  p.  68). 

Pyrophyllite  (p.  324)  and  gypsum  (p.  256,  and  Fig.  134, 
p.  68)  also  have  forms  that  are  made  up  of  radiating  or, 
better,  stellated  plates. 

Micaceous.— The  micas  (p.  303  et  seq.),  muscovite,  bio- 
tite,  phlogopite,  etc.,  separate  readily  into  thin,  usually 
tough,  flexible,  and  often  elastic  laminae  or  leaves. 

Clinochlore  (p.  311)  and  some  related  minerals,  com- 
monly green  in  color,  give  tough,  inelastic  laminae.  Some 
other  species  give  soft,  more  or  less  brittle  leaves,  but 
cleave  in  the  same  way  with  the  micas;  this  is  true  of  talc 
(p.  326),  brucite  (p.  260),  and  pyrophyllite  (p.  324).  One 
variety  of  gypsum  (selenite,  p.  256)  also  separates  by 
cleavage  into  soft,  brittle  laminae,  but  it  is  not  properly 
micaceous.  Orpiment  (p.  174)  yields  thin,  flexible  plates 
of  bright  yellow  color  and  brilliant  luster. 

Foliated. — Some  of  the  minerals  just  mentioned,  as  talc, 


354  MINERALS,  AND    HOW   TO   STUDY   THEM. 

pyrophyllite,  and  orpiment,  are  foliated  rather  than  mica- 
ceous. Graphite  (p.  168)  and  molybdenite  (p.  178),  among 
minerals  with  a  metallic  luster,  are  conspicuous  for  their 
foliated  character. 

Mammillary. — A  mammillary,  botryoidal,  or  globular 
surface  is  often  seen  with  prehnite  (p.  331,  and  Fig.  138, 
p.  68),  calamine  (p.  237),  smithsonite  (p.  237),  chalcedony 
(p.  278,  also  Fig.  137,  p.  68),  hyalite  (opal,  p.  283); 
hematite  (p.  217)  may  be  included,  though  a  reniform 
surface  is  for  it  particularly  characteristic  (Fig.  139,  p.  68). 
Limonite  (p.  222)  is  often  in  stalactitic  forms  (Fig.  140,  p. 
68);  also  gibbsite  (p.  241)  and  occasionally  marcasite  (p. 
215). 

CLEAVAGE. 

Cubic  Cleavage. — This  is  exhibited  conspicuously  by 
halite  or  rock-salt  (p.  268);  also,  among  metallic  species, 
by  galena  (p.  198,  and  Fig.  143,  p.  71). 

Anhydrite  (p.  258)  and  cryolite  (p.  242)  sometimes  show 
cleavage  in  three  directions,  which  sometimes  resembles 
cubic,  though  not  so  in  fact. 

Octahedral  Cleavage. — This  is  usually  very  distinct  with 
fluorite  (p.  245  and  p.  71). 

Dodecahedral  Cleavage. — This  is  characteristic  of  spha- 
lerite or  zinc  blende  (p.  233). 

Rhombohedral  Cleavage. — This  is  conspicuous  with  the 
species  calcite  (p.  247,  and  Fig.  144,  p.  72),  dolomite  (p. 
260),  siderite  (p.  223),  rhodochrosite  (p.  232). 

The  basal  cleavage  of  the  micas  (p.  303),  chlorites  (p.  310), 
brucite  (p.  260),  and  other  species,  having  on  this  account 


ON    THE    DETERMINATION    OF   MINERALS.  355 

a  foliated  or  "micaceous"  structure,  is  an  important  char- 
acter. Orpiment  (p.  174)  has  also  a  foliated  structure. 

Gypsum  (p.  256)  yields  large  thin  plates  by  cleavage. 

Topaz  (p.  320)  has  perfect  basal  cleavage.  Pyroxene 
(p.  292)  often  shows  a  basal  "  parting  "  resembling  cleavage. 

The  feldspars  (p.  284)  have  cleavage  in  two  directions  at 
right  angles  to  each  other,  or  nearly  so.  Corundum  (p. 
239)  sometimes  shows  a  rhombohedral  "parting"  resem- 
bling cleavage,  in  directions  inclined  about  94°  to  each 
other. 

Barite  (p.  262)  and  celestite  (p.  266)  have  basal  and 
prismatic  cleavage. 

Amphibole  (p.  296)  has  prismatic  cleavage  (124|°).  The 
scapolites  (p.  314)  have  cleavage  parallel  to  the  two  square 
prisms. 

Among  minerals  with  metallic  luster  the  following  have 
conspicuous  cleavage:  graphite  (p.  168),  stibnite  (p.  176). 

HARDNESS  AND  TENACITY.     (See  pp.  74-78.) 

Very  soft :  having  a  greasy  feel.  Here  belong  talc  (p 
326)  and  pyrophyllite  (p.  324);  also  kaolin  (p.  239). 

Graphite  (p.  168)  and  molybdenite  (p.  178),  among 
minerals  with  metallic  luster,  have  also  a  greasy  feel  and 
soil  the  fingers. 

Soft:  scratched  by  the  nail.  Here  belong  gypsum  (p. 
256),  brucite  (p.  260),  orpiment  (p.  174),  sulphur  (p.  170), 
cerargyrite  (p.  185),  cinnabar  (p.  187),  also  some  chlorite. 

Further,  among  minerals  with  metallic  luster  may  be 
mentioned:  stibnite  (p.  176),  color  lead-gray  and  luster 
metallic;  argentite  (p.  184),  sectile,  yields  silver. 


356  MINERALS,  AND   HOW   TO   STUDY   THEM. 

Hard  Minerals. — It  should  be  noted  that  most  of  the 
hard  minerals  belong  to  either  the  class  of  the  Oxides  or 
that  of  the  Silicates.  The  sulphides  are  mostly  soft,  that 
is,  H.  =  4  or  below;  the  exceptions  are  the  minerals  of  the 
Pyrites  Group,  of  which  the  common  members  are  pyrite, 
marcasite,  arsenopyrite  (pp.  213-215);  these  are  hard 
enough  (6  to  6.5)  to  scratch  glass.  Some  related  sulphides 
(and  arsenides)  of  cobalt  and  nickel  have  H.  =  5  to  55. 

The  Carbonates,  Sulphates,  Phosphates,  etc.,  are  also 
mostly  soft,  rarely  up  to  5.  Of  the  Silicates,  those  yield- 
ing water,  like  the  zeolites,  are  relatively  soft,  rarely  up 
to  6. 

Again,  another  distinction  partly  contained  in  the  above 
is  that  minerals  of  metallic  luster  are  not  often  hard.  The 
most  conspicuous  exceptions  are  the  members  of  the  Pyrites 
Family  among  sulphides,  alluded  to  in  the  paragrah  above, 
and  hematite,  magnetite,  franklinite  (pp.  217-221)  among 
oxides. 

The  following  minerals  are  hard,  mostly  falling  between 
6  and  7: 

Prehnite  (p.  331).  Rutile  (p.  209). 

Epidote    and    zoisite  (pp.     Cassiterite  (p.  207). 

316,  317).  Diaspore  (p.  240). 

Feldspars  (p.  284).  Chrysolite  (p.  312). 

Vesuvianite  (p.  315).  Sillimanite  (p.  324). 

Further,  spodumene  (p.  295),  axinite  (p.  325),  iridosmine 
(p.  183),  danburite  (p.  321),  chondrodite  (p.  326);  finally, 
cyanite  (p.  324),  H.  =  5  to  7.25. 


ON   THE    DETERMINATION  OF   MINERALS.  35? 

The  following  are  very  hard — hardness  equal  to  that  of 
quartz  or  greater  (H.  =  7  or  aoove): 

H.  H. 

Quartz  (p.  273),  7  Beryl  (p.  298),  7.5-8 

Garnet  (p   300),      6.5-7.5  Spinel  (p.  241),  8 

Tourmaline  (p.  317),  7-75  Topaz  (p.  320),  8 

Staurolite  (p.  325),     7-7.5  Chrysoberyl  (p.  242),  8.5 

Zircon  (p.  312),  7.5  Corundum  (p.  239),    9 

Andalusite  (p.  323),       7.5  Diamond  (p.  166),    10 

Also  the  rarer  species  boracite  (p.  262),  H.  =  7;  iolite 
(p.  322),  H.  =  7  to  7.5;  euclase  (p.  299)  and  phenacite  (p. 
300),  H.  =  7.5  to  8. 

Malleability. — The  minerals  which  are  malleable  include 
the  native  metals  (see  p,  78) ;  less  perfectly  so  argentite  (p. 
184),  cerargyrite  (p.  185):  these  last  are  conspicuously 
sectile.  The  rare  silver  minerals  hessite  (silver  telluride), 
petzite  (gold-silver  telluride),  are  somewhat  sectile. 

Talc  (p.  326)  and  orpiment  (p.  174)  are  flexible;  also 
some  mica  and  chlorite. 

SPECIFIC  GRAVITY.     (See  pp.  79  to  88.) 

The  importance  of  the  specific  gravity  as  a  character  ID 
the  determination  of  minerals  has  been  repeatedly  insisted 
upon,  and  the  subject  has  been  so  fully  discussed  on  pp.  79 
to  88  that  to  give  a  list  of  minerals  of  conspicuously  low 
or  high  density  would  be  unnecessary  repetition.  The 
student  should  in  this  connection  read  again  carefully  the 
pages  referred  to. 


358          MINERALS,  AND  HOW  TO  STUDY  THEM. 

LUSTER. 

Metallic. — A  metallic  luster  belongs  to  all  the  Native 
Metals,  as  gold,  silver,  copper,  etc. ;  also  to  many  of  the  Sul- 
phides, as  stibnite,  galena,  pyrite,  but  not  to  sphalerite,  cin< 
nabar,  and  some  few  others;  finally,  to  a  few  of  the  Oxides, 
as  magnetite,  hematite  (some  varieties),  ilmenite,  chromite, 
franklinite.  The  Silicates,  Phosphates,  Sulphates,  Carbon- 
ates, etc.,  have,  with  very  few  exceptions,  an  unmetallic 
luster. 

Outside  of  these  classes  the  only  minerals  having  a 
metallic,  or  in  most  cases  more  strictly  a  submetallic,  lus- 
ter are  a  very  few  rare  silicates  not  described  in  this  work ; 
also  the  species  columbite  (and  tantalite,  p.  224)  and 
wolframite  (p.  225),  briefly  mentioned,  and  a  few  others 
related  to  them. 

It  should  be  noted  that  all  the  sulphides  having  an  un- 
metallic luster  (sphalerite,  cinnabar,  etc.)  are  soft.  A 
hard  mineral  (H.  =  6  or  above)  having  an  unmetallic  lus 
ter  is  either  an  oxide  or  a  silicate.  There  are  only  one  or 
two  rare  exceptions  (as  boracite,  H.  =  7). 

Adamantine. — An  adamantine  luster  (as  explained  on  p. 
89)  belongs  to  some  hard  minerals,  as  diamond,  corundum, 
zircon,  cassiterite;  also  to  a  number  of  minerals  containing 
lead,  as  cerussite,  anglesite,  and  other  rarer  ones,  also 
cerargyrite,  cinnabar,  cuprite;  further,  some  light-colored 
specimens  of  sphalerite  and  titanite. 

A  metallic-adamantine  luster  belongs  often  to  pyrargy- 
rite  and  some  specimens  of  cerussite  and  cuprite. 

Resinous. — Sphalerite  (p.  233)  is  a  striking  example  ol 
resinous  luster;  many  Phosphates  belong  in  this  class. 


ON  THE   DETERMINATION  OF   MINERALS.  359 

Vitreous  or  Glassy.— Quartz,  beryl,  garnet  are  familiar 
examples  of  vitreous  luster;  most  silicates  belong  here. 

Pearly.— Talc  (p.  326)  and  brucite  (p.  260)  have  con- 
spicuous pearly  luster,  also  pyrophyllite  (p.  324)  in  foliated 
varieties. 

A  pearly  luster  is  noted  on  the  basal  plane  of  apophyl- 
lite  (p.  331),  also  on  the  side  planes  of  perfect  cleavage  of 
gypsum  crystals  (p.  256)  and  those  of  stilbite  (p.  337)  and 
heulandite  (p.  338).  Barite  (p.  262)  and  celestite  (p.  266) 
often  show  pearly  luster  on  the  basal  plane;  so,  also,  some 
kinds  of  feldspar.  Other  minerals  belong  in  this  same 
class. 

Silky. — Fibrous  gypsum  (p.  257)  and  fibrous  calcite  (p. 
249)  (each  called  satin  spar),  also  asbestus  (p.  297)  are 
good  examples  of  silky  luster. 

COLOR. 
(a)  Metallic  Luster. 

The  following  lists  may  be  helpful,  although  it  has  not 
been  attempted  to  make  them  complete: 

Silver-white  or  tin-white:  Native  silver;  arsenopyrite, 
cobaltite,  and,  further,  several  rare  minerals  containing 
cobalt  and  nickel;  also  native  antimony, arsenic, tellurium, 
and  some  compounds  of  tellurium,  as  the  tellurides  of 
gold  (sylvanite),  silver,  or  lead.  The  color  is  often  dull  on 
the  surface  in  consequence  of  tarnish. 

Steel-gray :  Stibnite,  oxides  of  manganese,  as  pyrolusite 
and  manganite;  native  platinum. 

Lead-gray :  Galena,  molybdenite  (both  bluish).   StibuiU 


360  MINERALS,  AND    HOW   TO    STUDY   THEM. 

is  also  often  lead-gray.  Argeutite  and  chalcocite  are 
blackish  lead-gray. 

Copper-red :  Native  copper. 

Bronze-red :  Bornite  (with  variegated  tarnish),  niccolite. 

Bronze-yellow :  Pyrrhotite,  millerite. 

Brass-yellow :  Chalcopyrite,  brittle,  dissolves  in  nitric 
acid.  Pale  brass-yellow,  pyrite;  also  still  paler,  marcasite. 
Millerite  (see  above)  has  more  of  a  bronze  color. 

Gold-yellow  :  Native  gold  (malleable). 

Black  or  nearly  so:  Tetrahedrite,  chalcocite,  graphite, 
magnetite,  hematite,  ilmenite,  and  sometimes  limonite; 
also  (luster  submetallic)  columbite,  wolframite. 

The  following  are  conspicuous  for  the  tarnish  (often 
bright-colored)  of  the  surface :  Bornite,  chalcopyrite,  tetra- 
hedrite,  hematite,  some  limonite. 

The  STREAK  is  to  be  noted  particularly  in  the  case  of 
some  minerals  with  metallic  luster.  The  majority  have  a 
streak  which  differs  but  little  from  black,  but  it  is  usually 
dull,  not  shining.  The  streak  of  hematite  is  brownish 
red,  of  pyrargyrite,  cochineal-red.  The  streak  is  bright 
and  shining  with  graphite  and  molybdenite. 

(b)   Vnmetallic  Luster. 

Colorless :  Quartz,  not  cleavable,  hard ;  calcite,  if  crys- 
tallized, shows  rhombohedral  cleavage,  soft;  gypsum,  very 
soft.  Also  cerussite  and  anglesite,  some  crystallized  varie- 
ties of  albite,  barite,  apatite. 

White :  Many  massive  minerals,  especially  the  feldspars, 
quartz,  calcite,  barite,  cerussite,  scapolite,  several  of  the 
zeolites,  calamine,  talc,  meerschaum. 


ON   THE   DETERMINATION   OF   MINERALS.  361 

Blue:  Azurite,  usually  dark  blue  to  blackish  blue;  also 
sapphire,  cyanite,  some  celestite,  lazulite,  lapis  lazuli.  One 
•variety  of  tourmaline  has  an  indigo-blue  color.  Amethyst 
and  some  fluorite  are  violet-blue.  Some  beryl  is  blue  or 
greenish  blue,  also  amazon-stone.  Turquois  varies  from 
robin's-egg  blue  to  greenish  blue  and  bluish  green.  Lapis 
lazuli  is  a  bright  blue.  Chrysocolla  and  some  varieties 
of  calamine  and  calcite  belong  here,  also  some  more  or  ^ss 
rare  copper  minerals  (as  chalcanthite  and  some  other  sul- 
phates, etc.). 

Green. — Emerald-green:  This  is  characteristic  of  some 
beryl  (emerald),  also  malachite,  dioptase, 
spodumene  (hiddenite);  also  some  other 
minerals  containing  copper,  most  of  them 
not  described  in  this  book  (as  atacamite). 

Bluish  green :  Much  beryl,  apatite,  fluorite, 
amazon-stone,  tourmaline,  chlorite,  prehnite, 
calamiue,  smithsonite,  chrysocolla. 

Apple-green:  Talc,  some  garnet,  chrysoprase, 
willemite,  nickel  silicate. 

Yellowish  green :  Some  beryl  and  apatite, 
chrysoberyl,  chlorite,  also  (olive-green)  chry- 
solite, datolite,  and  some  serpentine,  vesuvi- 
anite,  titanite.  Epidote  is  pistachio-green  ; 
pyromorphite  is  grass-green. 

Some  varieties  of  amphibole,  pyroxene,  also  serpentine, 
are  dall  grayish  or  blackish  green.  Wavellite  has  green 
varieties  of  several  shades.  Gahnite  is  dark  green. 


32  MINERALS,  AND   HOW   TO    STUDY   THEM. 

Yellow.— Sulphur-yellow  :  Sulphur,  some  vesuvianite. 
Orange-yellow:   Orpiment,  wulfenite. 
Straw-yellow,   also   wine-yellow,   wax-yellow: 
Topaz,  sulphur,  fluorite,  cancrinite,  wulfen- 
ite,   vanadinite,    willemite,    calcite,    barite, 
chrysolite,  chondrodite,  etc. 
Broivnish  yellow :  Much  sphalerite,  siderite. 
Ocher-yellow :  Yellow  ocher  (limonite). 
Red. — Ruby-red :  Ruby  (corundum),  ruby  spinel,  much 

garnet,  proustite,  vanadinite,  sphalerite. 
Cochineal  red :  Cuprite,  cinnabar. 
Orange-red :  Zincite. 
Crimson  -  red :    Tourmaline    (rubellite),    spinel, 

fluorite. 

Orange-red :  Realgar  (to  aurora-red). 
Scarlet-red :  Cinnabar. 
Brick-red:  Some  hematite  (red  ocher). 
Rose-red:  Rose  quartz,  rhodonite,  rhodochrosite, 

erythrite,  some  scapolite  and  apophyllite. 
Peach-blossom  red  to  lilac:  Lepidolite,  rubellite. 
Flesh-red:  Some  feldspar,  willemite  (the  variety 
troostite),  some  chabazite  and  stilbite,  apatite, 
less  often  calcite. 

Brownish  red:  Jasper,  limonite,  garnet,  sphaler- 
ite, siderite,  etc. 
Brown. — Reddish  broiun :  Some  garnet,  some  sphalerite, 

cassiterite. 

Clove-brown  :  Axinite,  zircon,  pyromorphite. 
Yellowish  brown :  Siderite,  sphalerite,  jasper, 
limonite,  goethite,  tourmaline. 


ON   THE    DETERMINATION    OF    MINERALS.  363 

Blackish  brown :  Titauite,  some  siderite,  spha- 
lerite. 
Smoky  brown:  Quartz. 

Slack :  Tourmaline,  black  garnet  (melanite) ;  also 
(mostly  greenish  or  brownish  black)  some  amphibole,  py- 
roxene, and  epidote;  further,  some  sphalerite  and  some 
kinds  of  quartz  (varying  from  smoky  brown  to  black); 
also  allanite,  samarskite.  Some  black  minerals  with  sub- 
metallic  luster  are  mentioned  on  page  360. 

The  STREAK  is  to  be  noted  in  the  case  of  some  minerals 
with  unmetallic  luster.  By  far  the  majority  have,  even 
when  deeply  colored  in  the  mass,  a  streak  differing  but 
little  from  white.  The  following  may  be  mentioned : 

Orange-yellow :  Zincite,  crocoite. 

Cochineal-red :  Pyrargyrite  and  proustite. 

Scarlet-red :  Cinnabar. 

Brownish  red :  Cuprite,  hematite. 

Brown :  Limonite. 

MAGNETIC  PROPERTIES.     (Seep.  96.) 

Magnetite  (p.  219)  is  always  strongly  magnetic,  so  that 
a  fragment  jumps  to  a  good  magnet  even  when  separated 
by  a  little  distance.  Pyrrhotite  (p.  212)  is  also  magnetic, 
but  not  so  strongly  so  as  magnetite;  the  test  often  requires 
some  care  and  the  use  of  small  fragments.  Some  native 
platinum  (p.  181),  especially  the  variety  containing  con- 
siderable iron,  is  also  magnetic. 

The  following  minerals  are  slightly  magnetic  in  some 
varieties:  Hematite,  franklinite.  This  seems  to  be  due  to 


364  MINERALS,  AND    HOW   TO   STUDY   THEM. 

an  admixture  of  a  little  magnetite,  for  when  pulverized  a 
little  magnetic  powder  can  be  separated. 

In  general  a  fragment  of  a  mineral  containing  iron 
becomes  magnetic  when  roasted  on  charcoal  or  when  held 
in  the  forceps  and  heated  in  a  strong  reducing  flame. 
Thus  pyrite,  arsenopyrite,  chalcopyrite,  yield  a  magnetic 
globule  on  charcoal;  also  a  splinter  of  iron  garnet  in  the 
forceps  fuses  to  a  black  bead  which  is  more  or  less  mag- 
netic. 


APPENDIX. 


THE  following  list  includes  the  names  of  the  species 
which  it  is  most  important  that  the  young  mineralogist 
should  have  in  his  collection;  they  are  printed  in  SMALL 
CAPITALS.  To  these  are  added,  in  ordinary  type,  a  number 
of  others  which  are  also  important  but  not  quite  so  much 
so;  they  may  well  be  present  in  the  cabinet  of  the  school 
or  academy. 


GRAPHITE. 

SULPHUR. 

Orpiment. 

STIBNITE. 

Molybdenite. 

SILVER. 

GOLD  in  quartz. 

An  ore  of  silver. 

CINNABAR. 

COPPER.   • 

Chalcocite. 

Bornite. 

CHALCOPYRITE. 

TETRAHEDRITE. 

CUPRITE. 

MALACHITE. 

Azurite. 

GALENA. 

PYROMORPHITE. 

Mimetite. 

Vanadinite. 


CERUSSITE. 

Anglesite. 

Wulfenite. 

CASSITERITE. 

Rutile. 

PYRRHOTITE. 

PYRITE. 

MARCASITE. 

ARSENOPYRITE. 

HEMATITE. 

MAGNETITE. 

Franklinite. 

Chromite. 

LIMONITE. 

SlDERITE. 

Colninbite. 

MlLLERITE. 

Niccolite. 

Garnierite. 

Manganite  (or  Pyrolusite). 

RHODONITE. 


366 


APPENDIX. 


Rhodocrosite. 

SPHALERITE. 

Zincite. 

Willemite." 

Calamine. 

SMITHSONITE. 

CORUNDUM. 

Spinel. 

Cryolite. 

FLUORITE. 

Wavellite. 

CALCITE  (several  varieties). 

ARAGONITE. 

APATITE. 

Anhydrite. 

Brucite. 

GYPSUM. 

DOLOMITE. 

BARITE. 

Witherite. 

CELESTITE. 

Strontianite. 

HALITE. 

QUARTZ  (several  varieties.) 

OPAL. 

ORTHOCLASE. 

ALBITE. 

Oligoclase. 

Labradorite. 

PYROXENE  (several  var.). 

Spodumene. 


AMPHIBOLE  (several  var.). 

BERYL. 

GARNET. 

MUSCOVITE. 

BIOTITE. 

Lepidolite. 

Clinochlore. 

Chrysolite. 

Zircon. 

Scapolite. 

Vesuvianite. 

EPIDOTE. 

Zoisite. 

TOURMALINE. 

Topaz. 

Titanite. 

Andalusite. 

Cyanite. 

STAUROLITE. 

TALC. 

SERPENTINE. 

Datolite. 

PREHNITE. 

APOPHYLLITE. 

Pectolite. 

NATROLITE. 

Analcite. 

CHABAZITE. 

STILBITE. 

Heulandite. 


If  the  student  limits  himself  to  small  specimens,  as  ad- 
vised on  page  13,  a  collection  including  the  species  men- 
tioned will  not  occupy  a  great  deal  of  space,  and,  if 


APPENDIX.  367 

desired,  can  be  purchased  at  no  great  cost.  From  time  to 
time  additional  specimens  can  be  obtained  by  exchange  or 
purchase. 

Of  the  minerals  in  the  above  list  the  following  are  most 
desirable  for  the  blowpipe  and  other  chemical  trials  de- 
scribed in  Chapter  IV.  Suitable  fragments,  of  the  needed 
purity,  can  be  obtained  for  a  very  small  expenditure  of 
money. 

Stibnite,  molybdenite,  an  ore  of  silver,  cinnabar,  chalco- 
pyrite,  tetrahedrite,  cuprite  or  malachite,  galena,  pyro- 
morphite,  cassiterite,  rutile,  pyrite,  arsenopyrite,  hema- 
tite Or  siderite,  millerite,  rhodonite,  sphalerite,  corundum, 
cryolite,  fluorite,  calcite,  apatite,  brucite,  barite,  celestite, 
orthoclase,  amphibole  (actinolite),  garnet  (almandite), 
tourmalme,  natrolite. 

Also  in  addition  to  these :  a  mineral  containing  lithium, 
as  either  lepidolite,  spodumene,  amblygonite,  or  triphylite; 
one  containing  cobalt,  as  linnaeite;  chromium,  as  chromite 
or  crocoite;  vanadium,  as  vanadinite;  uranium,  as  uranin- 
ite  (pitchblende)  or  autunite. 


GENERAL  INDEX. 


Acicular  crystals,  63,  352 
Acid  aud  salt,  112 
Adamantine  luster,  89,  167,  358 
Aluminium,  aluminum,  238;  test 

for,  135 

Amalgam,  186,  187 
Amorphous  structure,  63 
Antimony,    175;    tests  for,    133, 

142,  151 

Aragonite  group,  119 
Arborescent  structure,  61 
Arsenates,  112,  175 
Arsenic,  173;  tests  for,  133,  142, 

150 

Arsenides,  175 
Aslerism,  93,  308 
Atomic  weight,  102 

Barite  group,  119 

Barium,  262;  test  for,  133 

Basal  plane,  Base,  31 

Beryl loid,  37 

Bismuth,  177;  test  for,  146 

Blebby  bead,  135 

Blowpipe,  description,  123;   use 

of,  127 
Borates,  113 
Borax,  use  of,  136 
Boron,  test  for,  133 
Botryoidal  structure,  67 
Brittle  minerals,  78 
Bromides,  110 
Bunsen  burner,  122 
Burner  for  blowpipe,  122 

Cadmium,  test  for,  146 
Calcite  group,  119 
Calcium,  244;  test  for,  133 


Capillary  crystals,  63 

Carbon,  166 

Carbonates,  112 

Carlsbad  twin,  286 

Charcoal,  124 

use  of,  140 

Chemical  compound,  106,  109 
elements,  100 
formula,  104 
symbol,  100 
tests,  153 

Chlorides,  110 

Chlorine,  test  for,  133,  155 

Chromates,  112 

Chromium,  tests  for,  138,  139 

Classification  of  minerals,  118, 161 

Cleavable,  66 

Cleavage,  65,  70,  354 

Basal,  72,  354 
Cubic,  70,  354 
Dodecabedral,  71,  354 
Octahedral,  71,  354 
Prismatic,  72,  355 
lihombohedral,71,  354 

Closed  tube,  125,  147 

Cobalt,  228;  test  for,  138 
nitrate,  use  of,  135,  144 

Cohesion,  15,  70 

Collection  of  minerals,  11 

Color,  90,  359 

Color  of  blowpipe  flame,  133 

Columbates,  113 

Columbiura,  test  for,  139 

Columnar  structure,  67,  353 

Concentric  structure,  67 

Couchoidal  fracture,  74 

Concretionary  structure,  69 

Concretions,  69 

369 


370 


GENERAL   INDEX. 


Contact-twin,  57 

Copper,  188;  tests  for,  133,  138, 

145,  156 

Copper  chloride,  test  for,  133 
Coralloichil  structure.  69,  253 
Crypto-crystalline,  65 
Crystal,  detiuition,  14 

distorted,  48 

how  formed, 1 7 

twin,  5? 

Crystalline  structure,  63 
Crystallization,  systems  of,  21 
Cube,  22,  346 

Decrepitation,  131 

Deudrites,  69 

Dendritic  structure,  61,  69 

Density,  79 

Description  of  species,  158 

Determination  of  minerals,  339 

Dichroism,  95 

Dimorphous  compound,  120 

Dioxide,  108 

Distorted  crystals,  48 

Dodecahedron,  22,  348 

Domes,  42 

Double  refraction,  94 

Drusy,  63 

Ductile,  78 

Earthy  fracture,  74 
Effervescence,  155 
Elasticity,  78 
Elements,  chemical,  99 

equivalence  of,  105 
Etching,  64 
Exfoliation,  135 

Ferrornangnnese,  229 
Fibrous  structure,  67,  352 
Flame  coloration,  133 
Flame,  oxidizing,  128 
reducing,  128 
Flexible,  78 
Fluorides,  110 
Fluorine,  test  for,  153 
Fluxes,  125 

Foliated  structure,  67,  353 
Forceps,  124,  130 
Formula,  chemical,  104 
Fracture,  73 
Fusibility,  scale,  131 


Glass  tubes,  125;  use  of,  147 
Globular  structure,  67 
Gold,  179 

Goniometer,  contact,  47 
Granite  veins,  287,  b07 
Granular,  66 
Greasy  luster,  89 
Grouping,  irregular,  62 
parallel,  60 

Hackly  fracture,  74 

Hammer,  11 

Hard  minerals,  356,  357 

Hardness,  74,  355 

Heat,  95 

Hexagonal  prism,  36 

pyramid,  36 
Hexoctahedron,  25 
Hydrates,  116 
Hydraulic  lime,  252 
Hydrous  compounds,  116 

Impalpable  structure,  66 
Inelastic,  78 
Intumescence,  135 
Iodides,  110 
Iridescence,  93 
Iron,  210  ;  test  for,  138 
Isometric  system,  22 
Isomorphous  group,  119 

Lamellar  structure,  66 
Lamp  for  blowpipe,  122 
Lead,  197;  test  for,  144 
Left-handed  crystal,  275 
Lithium,  test  for,  133 
Luster,  88,  358 

Magnesium.  259  :  test  for,  136 
Magnetic,  96,  363 
Magnifying  glass,  344 
Malleable,  78,  357 
Mammillary  structure,  67,  354 
Manganese,  2^9 ;  test  for,  138 
Mercury,  186;  test  for,  148,  149 
sulphide,  test  for,  148 
Metallic-adamantine    luster,   89, 

358 

Metallic  luster.  88,  357 
Metals,  102,  107 
Metasilicates,  113 
Micaceous  structure,  67,  353 


GENERAL   INDEX. 


371 


Microcosmie  salt,  125,  139 
Mineral,  artificial,  7 

definition  of,  5 
Mineral  kingdom,  1 
Mineralogy,  science  of,  5 
Molybdates,  112 
Molybdenum,  178  ;  tests  for,  134, 

139,  146 

Mouoclinic  system,  44 
Monoxide,  108 
Mossy,  63 

Native  elements,  109 
Negative  element,  107 
Nickel,  226  :  test  for,  138 
Niobates,  113 
Niobium,  test  for,  139 
Non-metals,  102,  107 
Nugget,  gold,  180 

Octahedron,  22,  347 
Odor,  97 
Opalescence,  93 
Opaque,  93 
Open  tube,  125,  147 
Orthorhombic  system,  41 
Orthosilicates,  113 
Oscillatory  combination,  53 
Oxides,  110 
Oxidizing  flame,  128 

Parallel  grouping,  60 
Paramorph,  56 
Pearly  luster,  89 
Penetration-twin,  57 
Percentage  composition,  116 
Peroxide,  108 
Phosphates,  112 
Phosphorescence,  94,  153 
Phosphoric  acid,  test  for,  133 
Pinacoids.  42.  44 
Plaster  of  Paris,  257 
Platinum,  181 

Wire,  125,  136 
Play  of  colors,  93 
Polysynthetic  twinning,  59 
Positive  element,  107 
Potassium,  268  ;  test  for,  133 
Prisms,  31,  32,  41,  44,  349 
Protoxide,  108 
Pseudomorph,  55 
Pyramid,  hexagonal,  36,  349 


Pyramid,  rhombic,  42 

square,  31,  32,  349 
Pyritohedrou,  30,  213,  348 
Pyro  electricity,  97,  318 

Quartzoid,  274 

Radiated  structure,  67 
Reducing  flame,  128 
Refraction,  double,  94 
Reniform  structure,  67 
Resinous  luster,  89,  358 
Reticulated,  63 
Rhombic  prism,  41,  351 

pyramid,  42 

Rhombohedral  system,  39 
Rhombohedrou,  39,  351 
Right  -handed  crystal,  275 
Roasting,  139 

Salt  of  phosphorus,  139 
Salts,  112 
Saturation,  137 
Scale  of  fusibility,  131 

hard  ness, '75 
Scale  IK  >hedron,  39,  351 
Secondary  twinning,  60 
Sectile,  78 

Selenium   tests  for,  134,  146,  152 
Semi-metals,  103 
Semi  transparent,  93 
Sesquioxide,  108 
Silica,  272  ;  test  for,  156 
Silicates,  113,  272 
Silicon,  272 
Silky  luster,  89 
Silver,  183  ;  test  for,  145 
Snow-crystals,  17 
Soda,  use  of,  140  ;  on  charcoal, 

144,  145 

Sodium,  268  ;  test  for,  133 
Soft  minerals,  355 
Solubility  in  acids,  154 
Specific  gravity,  79.  80,  357 

balance,  81,  82 
Spelter,  233 
Sphenoid,  36 
Spiegeleisen,  229 
Splintery  fracture,  74 
Square  prisms.  31,  32,  349 

pyramids,  31.  82,  349 
Stalactitic  structure,  69 


372 


GENERAL   INDEX. 


Stalactite,  250 

Tetrabexabedron,  25 

Stalagmite,  250 

Test-paper,  126 

Stellate  structure,  67 

Tin,  206  ;  test  for,  145,  146 

Streak,  92 

Titanium,  208  ;  test  for,  139 

Striations,  52 

Translucent,  93 

Strike-figure,  305 
Strontium,  266  ;  test  for,  133 

Transparent.  92 
Trapezohedron,  23 

Sublimate,  142,  148 

Triclinic  system,  45 

Subtrauslucent,  93 

Trigonal  prism,  317,  350 

Sulphates,  112;  test  for,  146 

Trisoctahedrons,  24 

Sulphides,  109 
Sulphur,  168  ;  test  for,  142,  146, 

Tuugstates,  112 
Tungsten,  259 

147,  148 

Twin  crystals,  57 

Symbol,  chemical,  100 
Symmetry,  defined,  27 

Twinning  axis,  57 
plane,  58 

planes  of,  27,  35 

Twinning,  polysyntbetic.  59 

System.  Hexagonal  36 

secondary,  60 

Isometric,  22 

Mouoclinic,  44 

Uneven  fracture,  74 

Orthorhombic,  41 

Un  metal  lie  luster,  89 

Rhombohedral,  39 

Uranium,  210  ;  tests  for,  138,  139 

Tetragonal,  31 

Triclinic,  45 

Vanadates,  112 

Vesicular  bead,  13o 

Tabular  crystals.  352 

Vitreous  luster,  89,  359 

Tautalales,  113 

Tarnish,  91,  92 

Water  of  crystallization,  116 

Taste,  97 

Water,  test  for.  152 

Tellurium,    173  ;    tests   for,  134, 
152 

Waxy  luster,  89 

Tenacity.  77 

Zinc,  233  ;  test  for,  143 

Tetragonal  system,  31 

Zircouoid,  34 

Tetrahedron,  29,  348 

INDEX  TO  MINERAL  SPECIES. 


Acadinlite,  337 

Achroite,  ixir    Tourmaline,  317 

Actinolite,  297 

Adamantine  spar,  240 

Adular,  Adularia,  287 

Agaric  mineral,  251 

Agate,  279 

Alabandite,  232 

Alabaster,  257 

Albite,  288 

Alexandrite,  242 

Allauite,  317 

Almandine,  Almandite,  302 

Altaite,  110 

Alum,  Native,  244 

Alumina,  239 

Aluminite,  244 

Aluminium  carbonate,  244 
fluoride,  242 
fluo  silicate,  320 
hydrate,  241 
oxide,  289,  240 
phosphate,  243,  244 
silicate,  323,  324 
sulphate,  244 

Alunite,  244 

Amalgam,  187 

Amazon-stone,  288 

Amber,  170 

Amblygonite,  244 

Amethyst,  277 

Oriental,  240 

Amianthus,  v.  Serpentine 

Amphibole,  296 

Aualcite,  Analcime,  336 

Anatase,  v  Octahedrite,  208 

Audalusite,  323 

Aiidrsdite.  302 


Auglesite,  205 

Anhydrite,  258 

Ankerite,  261 

Anorthite,  290 

Anthracite,  170 

Autiuiouite,  v.  Stibnite,  176 

Antimony,  Gray,  v.  Stibnite,  11 

Native,  175 
Antimony  glance,  176 

sulphide,  176 
Apatite,  254 
Apophyllite,  331 
Aquamarine,  299 
Aragonite,  252 
Argentiferous  galena,  199 
Argentine,  var.  Calcite,  247 
Argentite,  184 

Arkansite,  -car.  Brookite,  209 
Arragonite,  v.  Aragonite,  252 
Arsenates,  175 
Arsenic,  Native,  173 

Red,  174 

Yellow,  174 

White,  175 
Arsenic  oxide,  175 

sulphide,  174 
Arsenides,  175 
Arsenolite,  175 
Arseuopyrite,  215 
Asbestus,  297,  328 

Blue,  298 

Asparagus-stone,  255 
Asphaltum,  170 
Atacamite,  197 
AugU  e,  294 
Auriferous  pyrites,  214 
Auripigmentum,  174 
Autuuite,  210 


374 


INDEX   TO    MINERAL    SPECIES. 


Aventurine  quartz,  278 
Axinite,  325 
Azurite,  196 

Barite.  262 

Barium  carbonate,  265 
sulphate,  2ti2 

Baryta  =  Barium  oxide,  v.  Ba- 
rium, 262 

Burytes,  v.  Barite,  262 
Barytocalcite,  265 
Basjinite,  280 
Bauxite,  Beauxite,  241 
Bell- metal  ore,  v.  Stauuite,  206 
Beryl,  298 

Beryllium  aluminate,  242 
phosphate.  300 
silicate,  298,  300 
Beryllonite,  300 
Bismuth,  Native,  178 
Biotite.  308 
Bismuth  sulphide,  178 
Bismuthinite,  178 
Bitter  spar,  v.  Dolomite,  260 
Bitumen,  170 
Bituminous  coal,  170 
Black  lead,  169 
Black  jack,  234 
Blende,  234 
Bloodstone.  280 
Blue-John,  246 
Blue- vitriol.  197 
Bog  iron  ore,  223 
Bog  manganese,  231 
Boracite,  262 
Borax,  270 
Boruite,  190 
Bort,  var.  Diamond 
Bournouite,  200 
Braunite,  231 
Breithauptite,  228 
Breunerite,  262 
Brittle  mica,  310 
Brochantite,  197 
Bronzite,  295 
Brookite,  209 
Brown  coal,  170 

hematite,  222 
Brucite,  260 

Cacholong,  var.  Opal 
Cadmium  sulphide,  238 


Cairngorm  stone,  277 

Calamiue,  237 

Calc  spar,  v.  Calcite,  247 

Calc  sinter,  251 

Calcite,  247 

Calcium  arsenate,  259 

boraie,  259 

boro-silicate,  329 

carbonate,  247,  252 

fluoride,  245 

phosphate,  254 

silicate,  295 

sulphate,  256,  258 

tantalate,  259 

titanate,  259 

tungstate,  258 
Cancriuite,  292 

Capillary  pyrites,  t>.  Millerite,  227 
Carbon,   166 
Carnelian,  279 
Cassiterite.  207 
Cat's-eye,  242,  278 
Cat's  gold,  306 
Cat's  silver,  306 
Celestite,  Celestine,  266 
Cerargyrite,  185 
Cerium  phosphate,  271 
Cerussite,  203 
Clmbazite,  336 
Chalcanthite,  197 
Chalcedony,  278 
Chalcocite,   190 
Chalcopyrite,  191 
Chalk,  tar.  Calcite,  247 
Chalybite,  t.  Siderite,  223 
Chert,  280  • 
Chiastolite,  55,  323 
Childrenite,  225 
Chlor-apatite,  255 
Chlorite  Group,  310 
Chloritoid,  310 
Chlorophaue,  247 
Chondrodite,  326 
Chromic  iron,  221 
Chromite,  221 
Chrysoberyl,  242,  299 
Cl.rysocolla,  197 
Chrysolite,  312 
Chrysoprase,  280 
Chrysotile,  328 
Cinnabar,  187 
Cinnamon-stone,  301 


INDEX   TO    MINERAL   SPECIES. 


375 


Clay,  239 

Diaspore,  240 

Clinuchlore,  511 

Diopside,  293 

Coal,  170 

Dioptase,  197 

Cobalt  bloom,  228 
arsenate,  238 

Dipyre.  315 
Dislheue,  v.  Cyanite,  324 

arsenide,  228 

Dog-tooth  Spar,  249 

sulphide.  228 

Dolomite,  260 

Cobaltine,  Cobaltite,  228 

Dry-bone,  237 

Coccolite   294 

Colestiue,  v.  Celestite 

Eisenkiesel,  v.  Quartz,  62 

Coluinbite.  224 

Eisenrose,  62 

Copper,  Emerald,  197 

Elteolite,  292 

Gray,  193 

Emerald,  299 

Native,  189 

Emerald  copper,  197 

Purple,  191 

Emery,  239 

Red,  194 

Eustatite,  295 

Yellow,  191 

Eosphorite,  225 

Copper  arseuate.  197 

Epidote,  316 

carbonate,  195,  196 

Epsom  Salt,  Epsoruite,  262 

chloride,  197 

Erubescite,  190 

glauce,  190 

Erythrite,  228 

iiickel,  227 

Essonite,  v.  Hessouite,  301 

oxide,  194 

Euclase,  299 

phosphate,  197 

Eulytiue,  Euiytite,  303 

pyrites,  191 

silicate,  197 

Fahlerz,  v.  Tetrahedrite,  198 

sulphate,  197 

False  galena,  234 

sulphide.  190,  191 

lead,  234 

vitriol,  197 

topaz,  277 

Copper  ore,  Red,  194 

Feldspar  Group,  284 

Yellow,  190 

Fibrolite,  v.  Sillimauite,  324 

Copperas,  226 
Cordierite,  322 

Fire-opal,  283 
Fleches  d  'amour,  209 

Corundum,  239 

Flint,  280 

Crocidolite,  298 

Flos  ferri,  253 

Crocoite,  Crocoisite,  202 

Fluor-apatite,  255 

Cross-stone,  325 

Fluorite,  245 

Cryolite,  242 

Fluor  spar,  245 

Cuprite,  194 

Frankliuite,  221,  236 

Cyanite,  324 

Gahnite,  236 

Danaite,  228 

Galena,  Galenite,  198 

Danalite,  303 

Garnet,  300 

Dan  bu  rite,  321 

Garnierite,  228 

Datholite,  Datolite,  329 

Genthite,  228 

Dawsonite,  244 

Gey  se  rite,  283 

Deraantoid,  302 

Gibbsite,  241 

Derbyshire  spar,  v.  Fluorite,  245 

Glauber  salt,  270 

Descloizite.  202 

Gluuberite,  270 

Desmiue,  337 

Glaucodot,  228 

Diallage,  294 

Glimmer,  v.   Mica,  303 

Diamond,  166 

Gmeiinite,  337 

376 


INDEX    TO    MINERAL   SPECIES. 


Goethite.  223 
Gold,  179 

Iron,  arsenate,  225 
arsenide,  215 

Gold  telluride,  179 
GOthite,  223 

carbonate,  223 
columbate,  224 

Graphic  tellurium,  181 
Graphite,  168 
Gray  antimony,  176 
copper,  193 

uiobate,  224 
oxide,  217,  219,  222,  228 
phosphate,  225 
sulphate,  225 

Greenockite,  238 

sulphide,  212,  213,  215 

Grenat,  v.  Garnet,  300 

tantalate,  224 

Grossularite,  301 

Iron  pyrites,  213,  215 

Guano,  255 

White.  215 

Gypsum,  256 

Jade,  297 

Halite,  268 

Jadeite,  297 

Harmotome,  337 

Jamesonite,  200 

Hauerite,  232 

Jasper,  280 

Hausmannile,  231 

Jasper-opal,  283 

Haydenite,  337 

Job's  tears,  312 

Heavy  spar,  262 
Heliotrope,  280 

Kaolin.  Kaolinite,  239 

Helvite,  303 
Hematite,  217 

King's  yellow,  174 
Kyaiiite,  v.  Cyanite,  324 

Brown,  222 

Herderite,  300 

Labradorite,  291 

Hessouite,  301 

Labrador  feldspar,  291 

Heulandite,  338 

Lapis-lazuli,  291 

Hidden  ite,  295 

Laumonite,  Laumoutite,  33£ 

Hornblende,  297 

Lazulite,  243 

Horn  silver,  185 

Lead,  Argentiferous,  199 

Hornstoue,  280 

Black,  169 

Horse-flesh  ore,  191 

Native,  198 

Humite,  326 

Lead  carbonate,  203 

Hyalite,  283 

chromate,  202 

Hyalopl.ane.  285 
Hydraulic  limestone,  252 
Hypersthene,  295 

molybdate,  202 
phosphate,  200 
sulphate,  205 

sulphide,  198 

Ice,  172 

telluride,  110 

Ice-stone,  242 

Lepidolite  309 

Iceland  spar,  249 

Lepidomelane,  308 

Ichtliyophthalmite,  832 
Idocrase,  315 

Leucite,  291 
Libethenite,  197 

Ilmeuite,  222 

Lime  =  Calcium  oxide,  244 

Indicolite,  var.  Tourmaline,  317 

Limestone,  250,  251 

Infusorial  earth,  283 

Limonite,  222 

lolite,  322 

Linnoeite,  228 

Iridosmine,  183 

Lithia  mica,  309 

Iron,  Magnetic.  219 

tourmaline,  319 

Meteoric.  211 

Lithiophilite,  225 

Native,  211 

Love's  arrows,  209 

Olitfist  (hematite),  217 

Lumachelle,  250 

INDEX   TO    MINERAL   SPECIES. 


37? 


Lydian  stone,  280 

Milller's  glass,  283 

Made,  323 

Native  antimony,  175 

Magnesia  =  Magnesium     oxide, 

arsenic,  173 

259 

bismuth,  177 

Magnesite,  260 
Magnetic  iron  ore,  219 

copper,  189 
gold,  179 

Magnetic  pyrites,  212 

iron,  211 

Magnetite,  219 

lead,  198 

Malachite,  Blue.  v.  Azurite,  196 

mercury,  187 

Green,  195 

platinum,  181 

Alalacolite,  v.  Diopside,  293 

silver,  183 

Manganese  carbonate,  232 

sulphur,  170 

oxide,  229,  230,  231 

Natrolite,  335 

silicate,  231 

Natron,  270 

sulphide,  232 

Needle  zeolite,  335 

Manganite,  230 
Marble,  250,  262 

Nepheline,  Nephelite,  291 
Nephrite,  29? 

Verd  -antique,  328 

Niccolite,  227 

Marcnsite.  215 

Nickel  antimonide,  228 

Margarite,  310 

arsenide,  227 

Marialite,  315 

silicate.  228 

Meerschaum,  329 

sulphide,  227 

Mt-ionite,  315 

Melanile,  302 

Ocher.  Brown   222 

Meianterite,  226 

lied,  218 

Menaccanite,  v.  Ilmenite,  222 

Octahedrite,  208 

Mercury.  Native,  186 

Oliiroclase,  290 

Mercury  chloride,  187 

Olivenite,  197 

sulphide,  187 

Olivine,  :!12 

Mesitine,  Mesitite,  262 

Onyx,  279 

Mica  Group,  303 

Opal,  282 

Microcline,  288 

Ophiolite,  328 

ilicrolite,  259,  271 

Oraugite,  314 

Milk  opal,  283 

Orpiment,  174 

quartz,  277 

Onhoclase,  285 

tfillerite.  227 
Miuietite,  201 

Oriental    amethyst,    ruby,    sap- 
phire, topaz,  240 

Mineral  coal,  170 

Ottrelite.  310 

oil,  170 

Ouvaroviie,  302 

wax,  170 

Ozocerite,  170 

Mirabilite,  270 

•lispickel.  216 

Pachuolite,  243 

Molybdenite,  178 
Molybdenum  sulphide,  178 
Monazite,  271 

Peacock  copper,  92,  190 
Pearl-mica,  «.  Margarite,  310 
Pearl-spar   =   Cryst.    dolomite, 

Moonstone,  287 

260 

Moss-agate,  279 

Pectolite,  333 

Mountain  cork,  297 

Pencil-stone.  327 

leather,  297 

Penninite,  311 

Muscovite,  304 

Pentlandite,  227 

Muscovy  glass,  304 

Peridot,  v.  Chrysolite,  312 

378 


INDEX   TO   MINERAL   SPECIES. 


Perofskite,  Perovskite,  259 
Petroleum,  170 
Pharmacolite,  259 
Pharmacosiderite,  225 
Phucoliie,  337 
Phenacue,  300 
Pbillipsite.  338 
Phlogopite,  308 
Finite,  307 
Pitchblende.  210 
Plagioclase,  289 
Plasma,  var.  Quartz,  273 
Plaster  of  Paris,  257 
Platinum,  Native,  181 
Plumbago,  168 
Plumose  mica,  306 
Polianite,  230 
Polydymite,  227 
Potassium  chloride,  270 
Potter's  ore,  200 
Prase,  280 
Precious  garnet,  302 

opal,  282 

serpentine,  328 
Prehnite,  331 
Proustite,  185 
Psilomelaue,  231 
Pucherite,  112 
Purple  copper  ore,  190 
Pycuite,  v.  Topaz,  320 
Pyrargyrite,  185 
Pyrile,  213 
Pyrites,  Arsenical,  215 

Auriferous,  214 

Capillary,    ».   Millerite, 

Cockscomb,  215 

Copper,  191 

Iron,  213 

Magnetic,  212 

Spear,  215 

White  iron,  215 
Pyrochlore,  271 
Pyrolusite,  229 
Pyromorphite,  200 
Pyrope,  302 
Pyrophyllite,  324 
Pyroxene,  292 
Pyrrhotite,  212 

Quartz,  273 
Quicksilver.  186 


Realgar,  174 
Red  copper  ore,  194 
hematite,  217 
iron  ore,  217 
ocher,  218 
silver  ore,  185 
zinc  ore,  236 
Resiu,  Mineral,  170 
Rhodoohrosite,  232 
Rhodonite,  231 
Ripidolite,  311 
Itock  crystal,  277 
meal,  251 
milk,  251 
salt,  268 

Rose  quartz,  278 
Rubelliie,  319 
Ruby,  Balas,  241, 

Oriental,  240 
Spinel,  241 
Ruby  silver,  185 

Rutile,  208 
Sahlite,  Salite,  293 
Salt,  Common,  268 
Samarskite,  224 
Sanidine,  287 
Sapphire,  240 
Sard,  279 
Sardonyx,  280 
Satin -spar,  250,  257 
Scapolite  Group,  314 
Scheelite,  258 
Scolecite,  Seolezite,  336 
Scorodite,  225 
Selenite,  256 
Sepiolite,  329 
Serpentine,  327 
Siderite,  223 
Silex,  v  Quartz,  278 
Silica,  272 
Siliceous  sinter,  283 
Silicined  wood,  281 
Sillimanite,  324 
Silver,  183 

Horn,  185 

Native,  183 

Ruby,  185 
Silver  chloride,  185 

sulph-antimonite,  185 
sulph-arsenite,  185 
sulphide,  184 


INDEX  TO   MINERAL   SPECIES. 


379 


Silver  glance,  184 

Siuter,  Siliceous,  283 

Smalt,  229 

Smaltite,  228 

Suiaragdite,      var.     Amphibole, 

296 

Smithsonite,  237 
Smoky  quartz,  277 
Soapstone.  3:27 

Soda  =  Sodium    oxide,    v.    So- 
dium, 268 
Soda  niter,  270 
Sodalite,  291 
Sodium  borate,  270 

carbonate,  270 

chloride,  268 

nitrate,  270 

sulphate,  270 
Spathic  iron,  223 
Spear  pyrites,  215 
Specular  iron,  217 
Sperrylite,  183 
Spessartite,  302 
Sphalerite.  233 
Spheue,  322 
Spiegeleisen,  229 
Spiuel,  241 
Spodumene,  295 
Stalactite,  250 
Stalagmite,  250 
Stannite,  206 

Staurolite,  Staurotide,  325 
Steatite,  327 
Stibnite,  176 
Stilbite,  337 
Stream-tin,  207 
Strontianite,  267 
Strontium  carbonate,  267 
sulphate,  266 
Sulphur,  Native,  170 
Sylvanite,  181 
Sylvine,  Sylvite,  270 

Tabular  spar,  296 
Talc,  326 
Tantalite,  224 
Tellurium,  Graphic,  181 

Native,  173 
Tennantite,  194 
Tenorite,  111 
Tetrahedrile,  193 
Thenardite,  270 


Thomsenolite,  243 
Thomsonite,  335 
Thorite,  314 
Tiger-eye,  278,  298 
Tin  ore,  Tin  stone,  207 

oxide,  207 

pyrites,  v.  Stannite,  206 
Titanic  iron,  222 
Titauite,  322 
Titanium  oxide,  208 
Topaz,  320 

False,  277 
Topazolite,  302 
Torbernite,  Torberite,  210 
Touchstone,  280 
Tourmaline,  317 
Tremolite,  297 
Tridymite,  272 
Triphylite,  225 
Triplite,  232 
Trona.  270 
Troostite,  236 
Turgite,  223 
Turnerite,  271 
Turquois,  243 

Ulexite,  259 
Ultramarine,  291 
Uraninite,  210 
Uranium,  210 

phosphate,  210 

mica,  210 
Uvarovite,  302 

Vanadiuite,  201 
Verd-antique,  328 
Vermiculite,  309 
Vermilion,  Native,  187 
Vesuvianite,  315 
Vitriol,  Blue,  197 
Viviauite,  225 

Wad,  231 
Water,  172 
Wavellite,  243 
Wernerite,  314 
White-lead  ore,  203 
Willemite,  236 
Witherite,  265 
Wolfram,  Wolframite,  225 
Wollasionite,  295 
Wood  opal,  283 


380 


INDEX  TO   MINEIIAL   SPECIES. 


Wood-tin,  207 
Wulfenite,  203 

Xenotime,  271 
Yttrium  phosphate,  271 

Zeolites,  333 
Ziuc  blende,  234 
carbonate,  237 


Zinc  ore,  Red,  236 

oxide,  236 

silicate,  236,  237 

sulphide,  233 

spinel,  236 
Zincite.  236 
Zircon,  312 
Zirconium  silicate,  313 
Zoisite,  317 
Zuuyite,  303 


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